ive recently considered building something similar to a peavey 6505.
what would the cost be of building such an amp?
More than buying a 6505 amp. Youre not going to save money building an amp. And probably want to start with something simpler.
I have built a 5W Vibro Champ and it cost around $900 USD.

I am building a Deluxe Reverb and it will easily be $1200 USD.

Building that Peavey will cost way more than buying one from Peavey.
Consider that the original manufacturer of any amp (or pretty much any product of any kind) pays probably 1/4 (or less) of the cost for the parts that you would.
Yep. Building your own amp is only cheaper if it's a copy of something antique or boutique. Oh course, there are other reasons to build your own amp.
Playing through an amp or guitar you made yourself makes you sound (to yourself) 10 times more badass.
-Fender American Stratt
-Tube Cricket
-Homemade 20 Watt Tube (pics+clips soon)
-MXR Stereo Chorus
-Holy Grail Reverb

Theres no sensation to compare with this, suspended animation, a state of bliss...
You'd probably break even with that amp at the price it costs to buy new, $1200.

Transformers are very expensive, and that's where most of your cost gets eaten up.

The only time it's worth it to build an amp (cost wise), is if it's a rare and expensive reissue (Marshall 18watt), or no longer made at all.