https://store.steampowered.com/api/appdetails?appids={APPID}
Steam store API has an endpoint api/appdetails that takes in appids parameter. The thing is, I would like to receive data about multiple apps. Judging for the appids parameter name, it can accept multiple values. Already tried appids=730,100, appids[0]=730&appids[1]=100 and apppids=730&appids=100, neither worked the way I'd like to.
According to: https://wiki.teamfortress.com/wiki/User:RJackson/StorefrontAPI, app ids as csv like your first attempt does work, you just need to set the filters parameter to price_overview. When I tried this it did indeed work, but I got no actual data back so I'm not sure how useful it is.