Well, I just did a completed listing search on ebay and came up with about 500 for an average price. They just aren't moving well at the moment. You can check for yourself by going to ebay and doing an advanced search. Type in Slingerland and check the completed listings box. That will give you ALL of the Slingerland products. You can then narrow your search from there. Be sure to note just the ones that actually sold. There's lots out there unsold with high price tags. I just saw a Buddy Rich kit complete go for just over 500. Dang. Wish I'd seen that one.
You're going to take a hit on your kit due to the mod. That's a real drag. If it was all original, it would go for more money. Because of the mod, you're going to lose a pretty good percentage.
Your best bet would be to either wait or trade it. There are people here that would more than likely want to trade you. That's a nice kit and some of us (ME!) are not collectors and don't mind a players kit. I could care less about the mod, myself. I like the idea of built-in versatility. Kinda cool. A more purist type will poo poo it and bathe after viewing the pics. Takes all kinds.