1905

What is a good price for an original 1918 SA 1905 with original finish, wood grips, and m3 scabbard?

As little as you can pay. There's several sites to calibrate your wallet with... Hayes Otoupalik Is pretty reasonable I find. Bayonets Joe could be said to be a bit high... http://www.stewartsmilitaryantiques....nets-us.7.html These guys are pretty reasonable too.

Around $250-275.

Really? For one in an M3 scabbard? Glad I got one when I did.

Price is dictated by condition and originality. An M1905 dated 1918 should have either a blued finish over an unpolished blade or a parkerized finish. The bayonet did not originally have an M3 scabbard. How much of the finish is left? Has it been sharpened. Are there any nicks on the blade edge or back. Are the handle panels worn smooth or do they still show the grooves from the machine used to shape them?

Here's one that is in fair to good condition. I think the price on this is a little high. I'd put more at around $200, tops.
WWI US Army M1905 Bayonet Marked SA 1917 w M3 Scabbard M1903 Springfield Rifle | eBay

M3 scabbards are more or less common. The money is in the bayonet.

Pictures would help to gauge the value.