cheapest way to keep spiders out of your house?

Cheapest is a hoover and a tube of caulk. Vacuum webs and egg sacs regularly, especially in corners and behind furniture, and seal the gaps round windows, doors and where pipes come into the house. They follow the insects, so cutting down on those helps too. Outside lights near doors off at night. Peppermint oil spray gets recommended all the time, I've never been convinced it does much.