How are you charging the current batteries? SLA/AGM batteries in 7-12ah are dirt cheap. That said, the new lithiums (marcum brute, amped outdoors, dakota, etc.) are very light. Keep in mind they require a lithium specific charger and have a different discharge curve compared to sla/agm. Some have a 12v start, with a bulk of their discharge curve being in the 11v range. You would likely want the 13v start. There are some great tests/comparison videos on youtube

Mat Peirce
1.25 acre southeast Iowa pond
LMB, BG, YP, WE, HSB, RES, BCP