If you buy a course without needing the VISA, you will often find a better price! they put that up rather quickly along with the ED Visa package, plus then they normally will do the 90 day Immigration office procedure for you too, this will come at an extra fee over and above the 1900 baht!. Plus then your Thai learning could be done with greater flexibility rather than being tied into it with an ED Visa... this is only my opinion based on personal experience of course :)
Personal experience and taking in what Tod has given us here, The non O attained in Savannakhet will be the cheapest! ED visa, the schools make a lot of money from you (leave it at that)