Hi,
Prices vary widely across different countries. We pay between $400 and $1500 per Gbit/s per month in "popular and cheap locations". In a scenario where we want to grow the network and at least keep the current geographical diversity (or even grow it), we'd have to at least equally strengthen less fortunate locations.
The list of the top 20 countries [0] contains countries such as Russia, Poland, Norway and Hungary, where international bandwidth is quite expensive. Hopefully someone from this list can help us compile or find estimates for pricing per country. It would be interesting to learn the prices even for mid-size (~50-100Mbit/s+) relays in any country outside of the top five countries, even more so for crazy places that don't make the top 20.
Another factor that we should not ignore is that, while it may be possible to find a set of ISPs that allow mid-size exit relays, it gets harder the more traffic you push, because of the additional workload (and scariness) for the ISP from complaints.
[0] https://compass.torproject.org/