Ok first off the term "last 17" really grades on me. Lumat and Paploo aren't that rare in the US and are part of the ROTJ line, rest are POTF figures.
I'm not sure if there's any good price guide sites any more. I use to go to something on Jedinet, but that's long gone. The problem with price guides is they tell you what somebody else was willing to pay for an item, not what the value of the item is. Those are two very different things. The general advice I have for anybody is the true value is what you're willing to pay for it, not somebody else.
The some thing I do a lot of is watching auctions. Unless it's a super rare item I'll watch to see what the going price is. It's a good way to get a feel of what I'll have to shell out. But you have to remember what an item goes one day is not what it's likely to go for the next. I'll put in a low bid on the off chance I win. It doesn't happen often, but does happen.
The best thing is to go with your gut. If it feels wrong walk away. I've past on lots of stuff due to price, shipping, location and seller. £25 is about $40. That's pretty high for a loose incomplete, common figure. POFT of figure maybe. Stormtrooper? Not for me.
If all else fails remember Yoda's advice, any of it.