In some locations it can be cheaper to rent from/to airport locations, like in your example where a

We are traveling to Andalucia from April 1-10. Our plan is to take the train from Madrid to Cordoba, then rent a car to drive to Granada-Ronda-Cadiz-Sevilla. We had planned on dropping the rental in Sevilla, then taking the AVE back to Madrid, though our Sevillano friend told us that doing a one-way rental would double reno hotel packages our cost of renting the car.
When I search on Kayak or AutoEurope, they mention drop off fees of about 50EUR, which isn't terrible and brings our total cost to about 250. Have any of you had trouble with one-way rentals? What rental company did you use? Does 250EUR for a 9ish day rental seem unreasonable? Lastly, (probably unlikely) was anyone able to rent a car with automatic transmission?
I would AVE Madrid-Seville, sightsee in Seville and then rent a car at Santa Justa to explore Cordova and Granada. Again HERTZ had incredible deals plus no drop-off charges to certain locations within Spain. I used to rent from Auto Europe. The second time they pulled a Bait Switch I was done with them (the first one I let it pass; the second reno hotel packages I got my money back thru Better reno hotel packages Business Bureau, though...).
includes: unlimited kms, one-way, one additional driver, full CDW/LDW w/ zero deductible (except for 36 handling fee per incident), airport fees if applicable, other fees, tank full out/in policy, free cancellation until 48hrs before rental starts
Rates vary, and included/excluded items vary widely so you must read the fine print to have a sound basis for price comparison. And some local or regional companies which pop up also with large consolidators like AutoEurope do charge for one-way rentals. But, as you discovered, those surcharges do not have to be excessively high.
In some locations it can be cheaper to rent from/to airport locations, like in your example where a rental from Cordoba airport to Sevilla airport would be 30 cheaper. IMO, not worth the hassle, though, since you will arrive/depart by train.