First of all did you know there is a permanent visa thread at top of branch - its too long to read but you can skim thru the last 6 or 7 pages.
But note that the 30 day stamp IS NOT A VISA - it has different conditions. Thai immigration routinely ignore these rules and providing your passport is in order its a defacto system of automatic entry with no questions asked. . And you should give your nationality and from WHERE you are flying from. ??????????
In a nutshell if you are flying from within the region airline staff are usually not that fussed and there can be possiblity of signing disclaimer / waiver form if they do object . If flying Europe or UK there are mixed reports about how strict airline staff are about enforcing these rules . From Australia airline staff are often strict although there was a recent report from someone who was worried about his onward ticket being outside the allowed 30 days , so he got another ticket that was within 30 days , but at the airport no one asked him for ticket at all and he threw away the ticket he specially bought.
The rules don't specifically say round trip , it can be just onwrad ticket although in rare caes when immigration does ask for this they have the power to specify where the ticket can be to . I know of one case of a person with prior drug convictions being made to get a ticket out of the region by Thai immigration , his ticket to KL wasn't good enough.
Note that cheap internet tickets such as Air Asia may not be refundable . Another option is to just get a visa or alternatively arrive at the airport early and see if they will allow you to fly , if there are problems offer to sign a disclaimer form , if that doesn't work offer to buy refundable ticket out at teh airport. But again WHERE are you flying from ?.