Well, nothing can replace real Mt. Dew for me, but I've taken a partial liking to Wal-Mart's "Dr. Thunder". It doesn't taste exactly like Dr. Pepper, but that's actually a good thing to me. I tend to have to .. use the restroom, after drinking too many 2-liters of real Dr. Pepper. I've been told that one of the "secret ingredients" is prune juice, which, based on my reaction to drinking lots of it, I tend to agree, actually. I don't have that issue with Dr. Thunder, although it's a bit too heavy on the carmel coloring, and the carbonation. Anyhow, the reason that I like it is - the taste is acceptable, and it's caffinated (!), and it costs half as much as the regular price for a 2-liter of Mt. Dew. (two for a 1$)
Barq's root-beer is good too (caffine!), but I can't drink more than two 2-liters of that stuff in a row. It's very.. oily, whatever they put in it. Possibly nutmeg or clove oil, I'm not sure.
Another "cheapy" soda that I've taken a liking to, is Price Chopper's "Black Cherry" soda. It's actually naturally, not artificially flavored, and it goes down pretty smooth. Don't think it's caffinated though. It too can be found on sale for $0.50 at times.
I go through about three 2-liters of beverages a day, so it helps to stretch the soda budget a little. I refuse to pay the "normal" highway-robbery price for Pepsi products, at $1.59 for a 2-liter.
PS. Stay far, far away from Stop 'N Shop's house-brand replacement for Mt. Dew. Tastes like artificially-flavored bannanas or something to me. Blech. I also don't recommend any of the alternative "cheapy" house-brand sodas at Wal-Mart or Price Chopper other than the ones that I've mentioned, although P.C.'s generic "Cola" isn't too bad, at $1.00 for a 3-liter, but I generally don't drink just regular "cola" drinks. (I likewise avoid Pepsi and Coke.)