This is just my opinion, but I think you were using a couple of excellent prarrie dog calibers in the 223 and 22-250. The 223 should handle those shots out to 300 yards and the other takes over, past that distance. The economics of squirrel or prarrie dogs dictates something cheap to load for, and you cannot beat the 223 for that application.
204 Ruger is something I have not played with. You are going to need the special things that seventeen shooters require, funnel, cleaning rod, etc. Cost per kill? Can't help you, but I suspect that it is a little more expensive to shoot; it may be inferior, or superior, ballistically? Beats me? Sure isn't a deer gun, to most people.
Looking for a dual purpose rig, right?
<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>What I really want is a caliber to shoot accurately 300 to 400 yds, use it for prairie dogs in the summer and deer in the fall. <HR></BLOCKQUOTE>
I think you would be very happy with a 243. Covers all your requirements, but it is not brand new and trendy. Even so, that's what I'd do.
Good hunting. LB