Please read this post:
If you don't mind the weight (about 3 lb), I recommend a Quantum Turbo. The battery cell is an sealed-lead acid 8V, 3.2Ah, commonly available for about $20. If maintained correctly (charged after each use, and not let to exhaust), a battery cell can last up to 10 years. Other Turbo models use either NiCd or NiMH cells, which make them lighter, but replacing the battery cells will be costlier (you may have to send it in for "professional" recelling).
The price of a decent Quantum Turbo with new cell on FleaBay is $140 - $160. If you search there for seller "BigSpecBear" (yes, that's me), I've been selling many of them recently.
Oh, you also need a power cable, model CM58 I think.