I recently purchased an Olympic Arms PCR rifle from a local shop. I'm not an expert on these fine rifles but I couldn't pass up a good deal. Here are some specs: Broached 16" 1x8 twist, Stainless Ultra Match barrel. Flat top Upper.Free-float hand guard, weaver gas block  All for $650. (It was a promotional thing) The more I read about this gun, the more I was bummed that I couldn't shoot the surplus. I know, I know, why would I shoot cheap ammo in an expensive gun?? I'm now broke, that's why. My question is, What benefits do I get from shooting .223 ammo? Accuracy? Also, is there any cheaper ammo that I could shoot in my .223 chambered rifle?

Have the chamber reamed to 5.56.  Ammo is much cheaper.  You lose some accuracy but you save tons of money.

Personally I have shot 500+ rounds of 5.56mm MilSpec ammo out of a .223 chamber with zero problems.If you want peace of mind, do what HiramRanger suggested.

Just my 2 pennies.

you upper is built to performace great accuracy. Don't alter it. Just shoot it. I am pretty sure it can eat surplus ammo and shoot well.

Reaming the chamber out to 5.56 would defeat the purpose of having the match barrel in the first place.  I wouldn't do it.

Try a little of the surplus and check carefully for pressure signs.  If you have problems, don't shoot it.  If not, go ahead and use it.  

For cheap .223 ammo try the 55gr FMJ Win. USA in the 40-round packs at Wal-Mart.  For cheap, more accurate ammo try the 45gr Win USA "varmint" loads, also from WM ($11/40), and the Black Hills 52gr/68gr/75gr match ammo ($16-$20/50).  

Quoted: Try a little of the surplus and check carefully for pressure signs.  If you have problems, don't shoot it.  If not, go ahead and use it.

Pressure signs?  Sorry, I'm a n00b, what exactly would you be looking for?