Doesn't make much sense to put a new header in and not do the exhaust. You're gonna be restricted at the exhaust and if you're gonna go through the trouble of cutting off exhaust bolts at the flex might as well do everything. The svt flex doesn't bolt up to the regular exhaust flange anyway unless you slot the bolt holes or replace the flange. Used Svt exhaust can be had for under $100 and it's 2 1/4 inch mandrel bent stainless steel. Unless you are putting in a turbo that will be plenty big.
And $500 is at the very low end if you can find everything used, but I suppose it's doable.