The best currency you can spend (after money for some gun oil, gun socks, dehumidifier, whatever you think is necessary, etc.) is time. On a regular basis -- I do it once a month -- actually inspect your firearms. Get a light and make sure there are no issues in the bore, the receiver, the bolt, etc. Make sure everything is properly oiled and lubed. Give them a wipe down with an oily rag or silicon impregnated cloth. The best preventative measure in the world is your Eyeball, Mark I.
I know a cat who took the usual steps and after a few months pulled out an older rifle and found rust in what was a previously clean bore. How did it happen? Who knows, they guy is fastidious in his cleaning but probably had a brain freeze the last time he took it out, fired corrosive through it and then improperly cleaned it. Although he managed to clean it, it's now a darker bore. Had he regularly checked everything over that wouldn't have happened.