Looks like the Airport or Carmel locations are 12-15 miles away.  Does anyone have a good take on how much that translates into for an Uber or Lyft?  I've seen estimates of $15 to $40.  Is the difference surge pricing or something?

Two $40 rides a day is a significant budget item.

Uber and Lyft each have a fare estimator on their websites. I've never used a ride share service in Indy but I've found the estimators fairly accurate when using them in other cities. I picked a hotel in Carmel (Springhill Suites by Marriott) and the Indiana convention center as an endpoint. It gives an estimate of $27 for Uber and $24-28 for Lyft. Both of those are using the least expensive car option. Here's the links to the fare estimators if you're interested in researching some price ranges for hotels.

https://www.uber.com/us/en/price-estimate/
https://www.lyft.com/rider/fare-estimate

I can't speak for Carmel, but I stayed at an airport hotel the first couple years I started regularly attending Gencon, and I never paid more than $15 (including at least a 15% tip) for a one-way Lyft ride. However, surge pricing can definitely affect things, so maybe I got lucky. Traditional cabs were about $22 for a one-way trip. If you are expecting at least $25/ride from Carmel and are able to drive, it may be worth doing the math to see if renting a car and reserving parking makes sense.

Just to add some info. Renting a car is about $385-450 for and economy class vehicle from Wednesday 3 pm to 8 am Monday. Gate 10 parking is about 80 bucks. 

That might help you in your calculations of deciding if Uber/Lyft is better than car rental.

yeah.  you can save money by getting a hotel further away and absorbing the uber expense.  still less money than a downtown hotel, but costs you some valuable convention time.