1st I would tell you that there is a difference in batteries, besides the $$. Even though there are only 3 battery manufacturers in the US who make the batteries for everyone from Walmart - Napa - Interstate, they are made to different specs. If an Interstate is $90 and the Walmart is $50...there's a reason for it!!!
If you want to save some $$ and still get a great battery, go to the site below and find your local Interstate distributor. If it's close enough to drive, give them a call and ask for a deep cycle "recon". Interstate recons vary from brand new from "buy-outs" to used that have been cleaned & tested, and they run from $20-$40. (sometimes they'll even have some deep cycle Optima batteries).
https://www.interstatebatteries.com