6s lipo pack
The pack will range from 25.2v fully charged ( 4.2v/cell ) to 21.6v discharged ( 3.6v/cell ). If there is a LVC, then this works perfectly.
Run two 6s hobbyking lipos in parallel and you'll have a 10ah pack for under $100.
Downside: you need to invest in the tools to charge and monitor lipo.