I am a big fan of Ultima Paint Prep Plus and Ultima Paint Guard Plus.
A 12oz bottle of Ultima Paint Prep Plus, costs about $26 and a 4oz bottle of Ultima Paint Guard Plus costs about $20. Although the initial outlay is around $60 after shipping, those two bottles will last quite a few years --or cover many cars, since you use only a tiny amount of each per application, and the results from these product last for months.
I performed a prep with Ultima Paint Prep and used a double application of Ultima Paint Guard Plus on our daily driver six months ago, and the water is still beading like it was recently applied --a sign that it is still protecting. That equates to about $10-12 a year if you try to push it. I know that there are many other products that come in at that price per year, however, I highly doubt any of them offer the quality of protection and reduced maintenance requirements, provided by these Ultima products.
Just a word of warning, apply Ultima Paint Guard Plus very thinly --one to two sprays on the applicator per panel. If you use too much of this product in one area, it may leave what appears to be a white haze on your paint. It I am performing double or triple layer application, I wait one day between applications.