Internet Exchange participants: how much do you pay?

I'm exploring starting a small IX in Montana for our local providers and large entities.

Currently, the only IX I personally participate in is the SIX in Seattle. There's no monthly recurring fee, only a one-time fee depending on the port speed.

I don't have a lot of datapoints, but IXs like Equinix have fairly high monthly fees to participate. Is this common? Or is the SIX method more typical?

I'd like this IX to be as cheap as practically possible because that encourages continued participation. At the same time, I can't imagine more than a dozen participants compared to the 300+ that SIX has, and that might be hard to sustain financially in a tiny market.