Going strictly by MSRP, a base , no-option 2010 Mustang GT was at most, $3400 less than a no-option 1SS (although the base GT was nowhere near as nicely equipped as the 1SS). However, after configuring a 2010 GT Premium similarly spec'd to my 2011 (Track pack, HIDs and Security Pacakge), and a similarly-equipped 2010 2SS/RS, the price difference was about $1300. Rebates, markups, whatever notwithstanding, I'm not seeing your '5 grand cheaper' here.
The price increase between the 2010 and 2011 (especially considering the new engines, transmissions and other refinements over the previous year) is not that great. A base 2011 GT is only $1250 more than a 2010 model. My 2011 GT Premium with the Brembo package, HIDs, Security package and 3.55s stickered at barely $2000 more than a comparable 2010. (However, I will admit, had Ford not put the 5.0L Coyote engine and a 6-speed manual transmission in the car and kept the car as-is for 2011, I'd most likely be in a 2SS right now [once I found a dealer that would've been willing to sell me one at or below invoice]).
Both cars have had their high (and low) points over the years; the various Shelby Mustangs, the Boss 302, 429 and 351, the 428CJ Mach 1 of the 60s and early 70s, the sadly forgotten SVO Mustangs of the mid-80s, the '87-'93 5.0s, 2000 Cobra R, '03-'04 Terminator Cobras to name a few, while the Camaro's history has the original '67-'69 big-block SS396s and 302-powered Z/28s, the various COPO cars, the '70 LT-1 Z/28 (my personal favorite classic Camaro), the later L98 and LB9 TPI 3rd gens, the 4th-gen LT1/LS1 Z28s and SSs, and of course the current 2010 Camaro to be rightly proud of.
But let's not forget the less-memorable cars from either camp. The '71-'73 Mustang ('71 Mach 1 and Boss 351 excluded, of course), the Mustang II (ugh), the early 80s 255 V8 cars that put out an awe-inspiring 117hp, the swarms of 2.3L 4-banger Fox-body cars, the '96-'98 2v 4.6L GTs and pre-fix '99 Cobras. I'm pretty sure Camaro guys would most likely rather forget the mid 70s Camaros (145-155hp 350 anyone?), the '82 Z28 with Cross-Fire injection, the 3rd-gen Iron Duke 2.5L 4-banger Camaros and of course, the '03-'09 Camaro.
Debating on which car has been 'much more' over the years is purely subjective (especially when both cars have had so many variations and have been around for 40+ years), and as such, is one which I will not touch with a 10-mile pole.
That said, with regards to the current GT and SS, I don't think there's been a Camaro and Mustang that has been this close, when objectively speaking from a performance and price standpoint. In my opinion, you can't go wrong either way.
