This is what I've experienced having owned 2 ATV's and one snowmobile. Bought new, the original batteries in all 3 machines lasted at least 6 years in each machine. When they finally died I bought suitable replacement batteries all cost at least $70. One purchased from CTC lasted less than 2 years before it died. The others lasted 2 seasons and both died. Dead died!
I replaced them all again but this time I picked up one of those $20 solar battery savers from CTC and it was well worth the investment. My current snowmachine battery is now on its 3rd year and shows no sign of quitting. My ATV, same story.