Hey Ted,
I've been shooting for a while now. (about 10 years)
The most bang for your buck IMO, still comes from the Ruger 10/22. It's infinitely customizeable, and even if left stock, is a very, very fun rifle. Can be had for about $300.00 or less.
A close second would be a classic lever action 1894 or similar cowboy rifle by Henry or Marlin in .22LR or .22mag. you can find these used, usually in good shape. New they're about $300-400.00. I'm likely going to start a collection of lever action rifles. I've found that they just make me smile to take them to the range. Probably a .22, a .357, a .30-30, and a .45-70 guide gun (for bear duty)
Next on my list would be a .22 upper on an AR15 lower. Several companies offer uppers now a days, that can be left on a dedicated lower (the registered part) or can share lowers with your .223 upper with some parts swaps. Prices differentiate on this. Lowers can be had from about $89.00-$200.00,(minus parts kit, grip, stock), and uppers run about $299-$500.00 depending on mfr. Count on at least $500 if you wait for the deals and gently used parts like stocks and grips, and about $800-$900.00 if you "just gotta have it now".
Finally, any of the fun EBR (evil black rifle) clones in .22 like the AK, thompson, or Hk Mp5 .22 would be very fun albeit a bit spendy. Not sure on current prices on each of these...