A Thompson was sold for around $225 in 1925, so adjusted for inflation that's roughly $3300. A good AR-15 style rifle will be around 1/3 of that, and an AK style rifle around $500.
I'm pretty sure those $225 Thompsons were fully automatic. You're not getting a full-auto AK for anywhere close to $500. Most full-auto weapons today are also in the $2000+ range.
