Mindo seems pretty far for a taxi. I think what you are really looking for is a private driver. Prices will vary based on vehicle used, etc but you can estimate it to be around $20/hour. Don't forget to factor in the driver's return time too. You are probably looking at around $80 to $100 for this transfer.
Taxis from the airport to Mindo generally are 70-100 dollars. If you like a taxi can be sent to the airport to pick you up and that way they will know exactly where to take you in Mindo. The price will be the same. Ask your hostel or wherever you are staying to arrange a taxi to pick you up at the airport. That is what we do with our guests. I feel much better if I know who our guests are riding with and that they will have no trouble finding us. Also, the driver will wait inside the airport to pick you up.
Let me know if you have any other questions that I can help you with,