Another vote for the Costco Callaway Hex Pro. I'm starting to improve, and not lose so many balls, so I wanted to try a better ball. I've tried the Bridgestone E6 and RXS, Srixon Z-Star and Q-Star, and Pro V1 and V1x.
For me, the Costco Callaway Hex Pro, at $24/dz, is a nice combination of quality ball and acceptable price. I don't feel awful when I still lose one, but the better feel is certainly noticeable around the greens.
Probably not for everyone, but for other price-conscious players looking to upgrade their ball, it's a fair compromise. I believe it's essentially a Callaway Tour iS from a few years back, with a urethane cover.