Do I need a return flight for my 90-day entry visa to Thailand?

If you are on a Tourist visa, a trip out of Thailand is required within the length of your Visa. I'm assuming 60 days + 30 day extension. So a ticket required within 60 days. You can book a cheap flight to Cambodia or Vietnam for around $50. There are other options . As often mentioned, Thai officers rarely ask for exit ticket. I've been asked maybe once, in many trips. However, airline gate personnel have asked me for an exit ticket on a few occasions. I recall one airline person actually counting the days on her fingers.

Which visa exactly are you entering with? If it's a non-immigrant visa, meaning the visa type starts with "Non-" then a one way flight should be fine.