OK, I'll jump in, in support of what spara50 says.
Since we're collecting for enjoyment, the cheaper the marbles, the further your money goes, and the more enjoyment you can get out of collecting.
For example, here's a fine set of German chinas with 6 limestone marbles and the bag their owner once used to keep them in. These possibly precede the invention of German glass marbles.
I got them on eBay last year for about 1/3 of what I would have had to pay for them at a show a few years ago, since a dealer would have most likely gotten hold of them before me and would have had to mark them up to cover his/her expenses and profit.