I feel like I'm always missing out on these deals.
I've been debating picking up Steel Battalion for awhile now, but I keep hoping I can pick it up for less than a hundred. Apparently you could do that a few months ago... not NEC related I know, but the current pricing climate has kept me from snagging things like Bonk 3 (still haven't played)... pretty sure I won't be playing Dynastic Hero anytime soon either.
I think 50-60 for a region modded console is fair. I think short of finding a really good deal, it's hard to get a TG-16 for less than 20-30 (some of that is just getting yourself ripped off, some Brick & Mortar specialty stores will charge you over a hundred for a core unit just because they're more rare than a Genesis/SFC - and eBay typically gets some inflation).
The mod itself has to be worth at least 20-30... which is still LOADS cheaper than finding a Kisado or some such, and if you found yourself a CD unit later, you could use the region mod to run cards like the Arcade card. I say has to be worth it from just a skill perspective. How comfortable are you cracking open your system and possibly ruining it? Would it be worth a Jackson to you to avoid doing it?
Anyway, the only reason I'd say it isn't worth a $100 or whatever is because so much of the appeal in TG stuff is for the CD games. It seems like as the price climbs, getting a Duo or some such would become the better deal (and wouldn't require a region mod for the CD games). Plus, you're still holding a system which has an RF connection, no save memory.. etc. That's just my opinion though, apparently guys like these don't have a problem charging a pretty penny for region modded core-type systems:
http://www.consolepassion.co.uk/pc-engine-consoles.htm