The rarest version of Slash is the bright yellow belly w/ purple S on belt. Loose and complete you might get about $25. You might get over $60 for one still on the card.
In general "loose" Ninja Turtles go for cheap now days. Most collectors are looking for unopened figures or at least complete ones. If you really want to get a good price for your lot, sell it together.
Some figures from the first series are worth a few more bucks than the comman variants (the 1st release of the 4 turtles). There were also a few from later in the line that are worth more like the foredog Hotspot and the cat Scratch.