So much of this depends on what type/quality of your camping gear. If you have good ultralight gear that is lightweight and compact, you can get away with fewer bags and a more inexpensive set up.

I strap this bag with Voile straps to my handlebars and have a secondary bag that goes on top with food and a foam pad up there too. On my seat tube, I have a compression sack with Voile straps like THIS. I am able to fit everything I need in these bags for a solid 2-night trip and it works really well for my uses. I have acquired really good used backpacking gear over the years though (sub 10lb weight) that makes this possible. I would check out r/ulgeartrade and craigslist for upgrade options.