Yes - the last bus leaves Marbella for Estepona at 23.15.
The last bus for Marbella leaves the Airport at 23.45.
Only options I could suggest:-
1. Take the bus to marbella then get a taxi from thee to Estepona.
2. Book into one of the budget hotels (Ibis, Novotel etc. - room cost around €60-70) near the Airport overnight then get the first Marbella bus the following morning which would still be slightly cheaper than the taxi fare.
3. Take the last bus to Marbella and pre-book a hotel there with late check in overnight before getting the first Estepona bus the next morning.
3. Camp out at the Airport overnight and take the first Marbella bus the next morning.
4. By far the easiest option - bite the bullet and take a taxi all the way from the Airport to Estepona.
Sorry that there does not seem to be any easier option.
Edited: 10:37 am, August 10, 2011