I don't know of any such site, but I can try to help with your question. Lanyards run around $8 (if I remember correctly). I am talking about the thicker pin lanyards sold at WDW. I'm not sure what you mean by glow merchandise, but if you are talking about the stuff that is lighted, you should expect to pay $5 - $10 for those items. We have purchased things like lighted mouse ears for around $10, those twirly character thingies for about $8 and lighted swords for around $5. We haven't purchased the glow necklaces, so I don't know what Disney charges for those. We purchased glow sticks (in the camping section) at Target and put it on a lanyard for my son when he was a little younger. I think we paid under $5 for a package of two.
I hope this helps!