You could also look in to Apriso, which is similar to Lialda (they are both one daily mesalamine drugs). Salix, the manufacturer, has a program that will reimburse you for your monthly co pay (up to $50 per month).
However, it kind of sounds like your insurance might only cover Asacol. Some insurances will only cover the least expensive form of a drug if there are several with the same active ingredient. Asacol works great, it's just a bit harder to use since you have to remember multiple doses over the day. I was doing fine on it, but chose to switch to Apriso for convenience.
Status: Remission since late May 2009!
Symptoms began in November 2008, ~4 weeks after giving birth to my son.
Diagnosed with pancolitis on 1/30/09
Currently taking Apriso (4 pills ONCE!! daily), Rowasa twice weekly, Natural Factors Ultimate probiotic 12/12 Formula, and Zoloft (25 mg).