What do you consider "affordable"? There aren't any genuine WW2 US rifles that routinely sell for less than $500, which based on your comment regarding M1 Carbines, seems to be what you're looking for.
The most widely-used US service rifles of WW2, including M1 Carbines, M1 Garands, M1903s, and M1903A3, are all typically $700+ in good condition. Sometimes much higher.
A Field Grade M1 Garand from the CMP would be worth considering. They're $630, but they'll be in somewhat rough condition. (Rack Grade M1 Garands from the CMP are even cheaper, but are very rough condition, and they're currently sold out of those anyway.) But it's worth the extra $100 for a $730 Service Grade M1 Garand, if you can afford it.
You might be able to find a postwar commercial copy of the M1 Carbine for around $500, like an Iver Johnson or Universal. It won't have the history of a real USGI carbine, and quality/reliability is hit or miss among all the various commercial manufacturers, but it could be close enough for display purposes.
M1917s tend to be the most affordable USGI rifles from that time period. You can sometimes find those in decent condition for $500-$600, if you keep your eyes open. They were used primarily during WW1, but they continued on in limited use during WW2, for training purposes and to arm second/third line troops.