I really haven't compared what modified components are selling for vs. unmodded gear, but have noticed most of it being in the digital area, which is notorious for losing value quickly anyway... Used high-end audio has always been a buyers market, also...
I would like to chime-in a few additional ramblings:
The mod business is still relatively new in the grand scheme of things, and many people aren't quite sure of what all this mod stuff is all about. To the average Joe, replacing a shitty cap, binding posts, diodes or resistors with good ones means exactly what? I think as we move forward, particular modification specialists will build their "brand" as more people talk about the difference particular mods have made to their gear, and as more audiophiles become educated on the value of each modder's work, resale value will strengthen based on overall value. Off the top of my head, Bill at Response, Dan at ModWright, Steve at Empirical, and Steve at Great Northern Sound have done alot to add credibility to this business, and each are pioneers in their own right... I know this because we often share the same customers, and word travels quick in this biz...
That said, I never have bought
ANY of my equipment based on what I expect to be able to sell it for down the road. At least it's not been a MAJOR consideration. I've always known that with the relatively high markups in the audio biz, I'm going to get bent-over either way

. Same thing goes with buying a car. No matter how great a deal I thought I got on a new or used auto, I always seem to take it in the fanny come time to trade in or sell. That's just life I guess. Unlike cars though, I make every audio purchase as if it is going to be my last. The eternal optimist, I guess
With mods, there's a disproportionately higher ratio of labor vs. parts, so you are buying expertise more than something tangible, and the markup on parts is relatively low, if anything at all. Hell, I probably pay my plumber more than what most these mod guys make per hour... Expertise is hard to put a value on, but my guess is this is no different than people modding cars, Harley's , or other exotica to get everything exactly the way they like it, and to extract every last bit of performance out of their gear. I spent an
entire summer modding a vintage 11 piece Ludwig Classic drumset that I paid $900 for. I stripped the crappy covering off with aheat gun and ALOT of patience, sanded the beautiful maple wood down smooth to the touch, took out all the lugs (holds the drum head to the shell) and stuffed with cotton to decrease vibration, re-dressed the drum bearing edges to make less contact with the drum head for more resonance, hand sanded and put several coats of natural Danish oil finish on them, then put at least 40 coats of tung oil/poly varnish on the maple shells with a 600 grit wet sand
between each coat. After owning these drums for 10 years, I decided to make the transition to electronic drums which utilize actual drum heads, and sell the old drums. I put these drums in the paper for $900 (the same amount I paid for them BEFORE all the work I did). I got 2 calls, and no sales. Does this mean I wasted all that time and money? I don't think so. Just because the lowballers don't see the value int hese drums doesn't mean these babies are worth less than I paid.... My guess is everybody here has a similar story...
The bottom line is that, I can say without any hesitation I'll take my ASL EXtreme Hurricanes for $11K before any other amp I've heard near that price. If and when I go to sell, some lucky schmuck is going to get a helluva deal, but that doesn't mean I got a shitty one... I'll just be grinning as I crank the tunes, and know that my Canes are as tricked-out as one of those masterpieces at Orange County Choppers...