Green Slag and Red Snarl are the really common ones. The third one of that set is Blue Grimlock, which doesn't show up as often, but still more often than the other colour versions of G2 Grimlock. Those where the three we got in Australia, so you could well find them quite easily at toyfairs, ebay and fleamarkets.
The regular silver/grey versions of each are probably quite common in America, but most ebay listings might not even mention it, unless they notice the G2 logo and name on the toy, plus the absence of a missle launcher. These three can still fetch a fair bit if the seller knows they are the G2 variant and has a buy-it-now on it. But I don't think auctions go too high, because the toys are pretty much the same as the Gen1 version, but would appear to be incomplete because the G2 versions only have a gun and sword.
As for the 3rd colour 'set', these are the rarest, and most expensive on ebay. The Green Snarl, Red Slag and Turquoise Grimlock were also only released in America, and like other low demand toys at their release, there are very few in circulation in the secondary market. Ones that do appear on ebay, can go US$50-100 each, and buy-it-now prices are US$200+ in packaging.
When I was looking for this rare set last year, I came across Grimlock fairly easily, but noticed since then, that one only appears every 3-6 months. The green snarl was about the same, in that I found one listed as a buy-it-now for about US$60 loose and no accessories (which was fine by me, because the weapons are the same as Gen1 Snarl, which I could buy cheaply from elsewhere). The unfortunate thing here though, was that I was getting desperate to complete my Gen2 collection last year, and bought this Snarl, knowing that it was from wheeljackslab (I had a dodgy toy from them before). And sure enough, the Snarl arrived missing a screw, so it arrived in two parts.
I still don't have a red Slag yet, and BotCon expenses in the last couple months have kept me from looking at Ebay (no point in finding something I can't afford), but during about 4 months at the end of last year, I only found one loose one (auction went to about $100), and a sealed one (buy-it-now at US$250).
So to summarise.
Least rare - Green Slag, Red Snarl, Blue Grimlock (released here), I'd be willing to pay up to AU$50 each complete. Dealer prices can vary, but auction prices would probably go to about US$50.
Medium rare (mmmm, medium rare (gurgle)) - Grey Slag, Grey Snarl, Grey Grimlock (not released here). I don't have these, because a small bit of paint with the G2 logo and their name is not a significant enough redeco for me to want them. As for value/prices, most of what I said above is guessed, because I was never really looking out for them in my ebay searches. And since I'd expect a lot of these being mistaken for Gen1 Dinobots, you could get one cheap (if you recognise it on an auction). Otherwise, you could end up finding them listed for high prices because of how few are actually correctly listed on ebay (just because there aren't many correctly listed as Gen2, doesn't mean there aren't many in the second hand market).
Really rare - Red Slag, Green Snarl, Turquoise Grimlock (not released here). I'm still looking for the Red Slag. I can't recall exactly what I paid for the other two, but should have been between US$50-100 each. Dealers will charge US$100-200+ for these figures, and auctions can get to US$100 depending on how many people see it at the same time. In other words, there is still very little demand for these figures, but their rarity will have the more dedicated collectors willing to bid more against others to get them.
Best thing is to do a weekly search on ebay US, searching for names that include the colours, or Gen2, their subgroup, or even just their names, as many sellers will just list the different colour versions by only their character name. After about a month, with the winning bids and buy-it-now prices recorded, you'll get an idea of what people are currently paying for each of them. And the current recession (especially in America), might see less competition in the auctions, compared to what I was paying above, just before the recession hit.