There are loads of buses that come to Mar del Plata and the price is approx. 100 pesos each way.
My favourite company is Plusmar because they have suites. My husband prefers Tony Turs because they give you Havanna alfajors (ha ha ha).
Some of the other companies are Chevalier, Flechabus, Impresa Argentino, Condor to name but a few. I have used all the companies and they are more or less the same. We have never had any problems with delays.
October is a lovely time to come here as it is still very quiet.