I'll start with the easiest stuff...
Your snare's definitely not a Slingerland. It's a low-end Japanese steel-shelled snare drum. Maybe Pearl, guessing by the lug style. Jonnistix, the forum's resident made-in-Japan guru, could probably give you a lot more info. The dollar value of this drum is low, if you can find a taker at all. You can find 'em all the time on eBay, but it seems very few of them actually get sold.
The hi-hat also looks Japanese. Again, Jonnistix could probably give you more info and tell you how many cents it's worth.
Moving on to the Slingerland drums... You've got a couple of strikes against you, starting with the fact you've got single-headed concert toms. There really just isn't a demand for those at the moment. (Things sometimes come back in vogue, so maybe someday...) A set similar to yours, but with a 24" bass and extra 18" tom was recently listed on eBay with a $480 starting bid, but there were no bidders. HERE is a link to the auction.
Because you've got four drums on two functional stands, I'd like to have these to use at the school where I work, but as true concert toms in a concert band setting, not on a drum set. Unfortunately, I can't make you an offer with the school district's money -- I'd have to buy them out of my own pocket. (Want your $50 investment back?) ;)
If I had to guess, I'd say you'd be likely to get somewhere in the $200's for the set. If you're including cymbals and stands and those cymbals are in the B8 family, add just a little bit. On the other hand, if you've got some pro-level cymbals, they could easily eclipse the value of the rest of the set.
And a couple little nitpicky things: That's not a white pearl finish. Pearl finishes have a sort of swirl pattern embedded within them -- check out this page to see what I mean. And at appears that one of your tom rims isn't original.