I don't know that this will necessarily answer your question regarding a cost-effective alternative to Hill's feline CD dry, but what follows is based upon my experience with my cat, Boo.
In November 2002, Boo blocked severely, having exhibited no previous problems. He spent about two weeks at the vet and ran up a bill of over $1,500. The vet sent me home with Hill's Feline CD dry and canned. He would not eat the canned food, so the dry CD became his sole food. I was not enamored with the ingredient list, but I put my trust in the vet and fed Boo the dry food exclusively until he had another bout almost exactly two years later. Luckily, he didn't block this time, but I went on a campaign to learn as much as possible about FLUTD and diet.
I am working from memory, but I recall reading a study of the effectiveness of the CD dry and canned in preventing a recurrence in cats prone to FLUTD. The canned product had a success rate several times that of the dry. So, I would conclude that the water content of the canned product had more of an effect on the ultimate outcome than the urinary acidifiers and low magnesium, etc. (common characteristics of both products). The ingredients in the canned CD are better than the dry, but they are still of inferior quality.
Another study states that cats fed canned food consume twice the water as those on a dry diet. I can confirm this firsthand. When Boo was eating nothing but CD dry, he would drink what seemed to me to be a significant quantity of water after eating, and I would see him get a drink or two at other times during the day also. Since December 2004, Boo has been eating canned food exclusively (half a 5.5 or 6 oz can in the morning and the other half can in the evening, with about 14 ml of distilled water stirred into the food at each meal). I have not seen him drink from a water bowl in months. I honestly think that I could dispense with the water bowls entirely. However, the interesting part is that I get almost exactly twice as much clumped urine out of his litter boxes (on a daily basis) as I did when he was eating dry food. It seems that my personal experience coincides with the above-quoted study. Anyone with a bladder problem has likely been told by the doctor to drink copious amounts of liquids. Most cats simply do not have a strong desire to drink, even when they are partially dehydrated; their brains simply aren't wired to compensate for an unnatural diet of dry kibble by drinking a large amount of water. A mouse is about 70-75% water, as are most canned cat foods, but dry foods are only about 10% water. Cats are designed to satisfy their water needs directly through their prey, with little need for supplementary drinking.
So, what does an increase in urine volume contribute to the equation? Firstly, the urine is less concentrated, making it more difficult for crystals to precipitate and either irritate the bladder wall or cause an obstruction. More dilute urine also means fewer bacteria and a shorter time in the bladder due to increased frequency of urination. The fact remains that a majority of cases of FLUTD are of unknown cause. More water definitely is a significant factor in prevention, however.
I am firmly convinced that dry kibble, prescrition diet or otherwise, is a very poor diet for a cat, especially one prone to FLUTD. I have taken a page from my study of fish nutrition (I wanted to be an ichthyologist once upon a time), and I am now feeding Boo the following canned food brands in rotation (a different brand each day): Natural Balance, Eagle Pack, Wellness, Innova, Nutro Natural Choice, Chicken Soup for the Cat Lover's Soul, Iams, Pet Guard, and Pet Promise. My feeling is that no one food, no matter how well-formulated, can be as good as a diet with some variety (i.e. there is no one perfect food). Any deficiencies or imbalances in any of the above brands will likely be cancelled out by the others. Also, a cat accustomed to this regimen is less likely to be finicky, which can be a consideration if a cat is fed a single food that has its formulation changed or is taken off the market, and they are also less likely to develop a food allergy. Since Boo has been on the canned diet, his activity and energy levels are noticeably higher. If cost is a consideration, consider Iams canned. It is not at the level of quality of most of the other mentioned brands, but it is a good food. I honestly think that even most of the supermarket brands of canned foods are better for cats than any dry food.
In addition to keeping cats in a semi-dehydrated state in which they are prone to FLUTD, dry kibble is also a recipe for feline diabetes. Not only is the water content unnatural, but the carbohydrate level in most dry foods is 35-50%, for a creature designed to consume prey that is about 5% or less carbohydrates. In fact, the cat needs, and, indeed, thrives on virtually no carbs in the diet. Here again, canned food is the winner because the carb level in a good canned food is about a fourth that of a good quality dry food. Likewise, canned food has it all over dry kibble with respect to protein quality and quantity.
I have probably gotten off-track a bit, but my recommendation is to ditch the CD dry (gradually) and get your cat onto a good canned diet-the best that your budget will tolerate (to which you stir in even more water). This is based upon my own experience and reading, and others might advise you differently. Ultimately, do your own research and trust your own instincts. Vets can be great resources, but never forget that they have a financial interest in selling you the foods that they carry (most of which have very poor quality ingredients in comparison with what you can buy at a good pet store). It doesn't take much research to come to the conclusion that dry kibble is a horribly unnatural, inappropriate diet for a cat. The fact that most do as well as they do on it is a testament to their adaptability.
Check out these resources:http://www.catinfo.orghttp://www.catinfo.org/zorans_article.pdfhttp://www.catnutrition.org/diabetes.htm