You might want to consider that record heat has been the norm this year in much of the outback, so pick your travel months carefully. The heat has caused the weather map to be extended with new colours as temperatures literally went off the chart (50C max or 122F was exceeded routinely).

Drop charges vary by rental company but tend to be zero between the major cities. Whether that extends to ASP is a good question. As for airfares, they don't vary much and are especially high for mining towns as there is huge demand (miners make $200K+ per year and fly in/fly out weekly). $234 is one of the more reasonable such fares, it can be $500 on some routes. As for the economics of driving, petrol costs $1.50 per liter or about $15 per 100km (60 miles) for a typical car, so ADL to Coober Pedy would be $120 for fuel. As for what else to see, Lake Eyre is wet again and pretty astonishing (though it might be dry by the time of your trip).