Can I get a STV and convert it to multiple entry later. + 30 day extension? Would it just be the price difference between the two?

 

I want the flexibility because I may be going to Japan around 60 days into me being in Bangkok, I would stay away for a week or two and then come back to Bangkok. 

I thought about just getting a multiple entry in the first place but I want to save the price difference if I end up not going to Japan.

 

 

A STV (special tourist visa) allows one 90 day entry that can be expended twice for 90 days. If you wanted to travel during the first 90 days or one of the extensions you could apply for a single re-entry permit  at immigration keep the remainder of the 90 days valid when you enter the country.

A METV (multiple entry tourist visa) allows unlimited 60 day entries for 6 months from the day it issued by Thai embassy or official consulate.

A single entry tourist visa (SETV) allows one 60 day entry that can be extended for 30 days.

An SETV (single entry tourist visa which is what I assume you mean) cannot be converted into another tourist visa. If you want another tourist visa, it is a new application. What is your nationality? If you are from a country that has transitioned to the eVisa system, you can apply for a new SETV while still in Thailand, and use it on your return from Japan.