I'd say a complete, 100% functional 6100/66 is worth around $25 to an obsolete Mac afficionado, more if it comes with original boxes/discs/manuals (which are somewhat rare if you are looking for them). It's pretty much worthless unless you are into old Macs - imagine a contemporary PC, a 486DX2/66...not worth much these days beyond recycling value.
In general old Macs aren't valuable, with a few exceptions - less common Macs like the TAM, Mac TV, 128k, or Macs that were originally super expensive like the IIfx, Apple Workgroup Server, 128k etc may command somewhat of a collector premium for the right buyer - but we're still usually talking under $100 (except the TAM). Also prototypes or non-US machines (like the Color Classic) have extra value.
But your average pre-G3 Mac will often be worth less than it costs to ship to a buyer.
Right now you can pick up 512ks, Pluses and SEs pretty damn cheap, and those early all-in-one Macs are going to be the first to really get collector value, so if you actually plan on buying an old Mac as an investment or collectable I suggest starting with one of those (or a 128k if you can get one).
The only Apple/Mac computers I've ever seen to go for big money (four figures) on ebay were an Apple I, a 128k in fantastic shape with all original packaging/manuals/disks/reciept, and a sealed-in-the-box TAM.
