For the unit that comes with the rotrex kit there is a 9.49 ratio. So, a little over 10.5k shaft rpm to hit its 100krpm max impeller speed.
Yes it is a little low in comparison, but there are still rc caliber motors that will work well in that range without too much gearing. Even if I have to gear it from say 20k down to 10, thats not bad. I haven't looked into it but I feel anything EV related will be significantly more expensive. But, I could be wrong.
There is a pretty good supply of sub 200kv brushless motors designed to replace small (50-150 cc) gas motors for various r/c aircraft.
http://www.rotrex.com/Home/Technology/Product_Overview