I am here with my family (3 of us) from the UK. We are all under 50.

Our 60 day tourist visas are about to run out. I went to the immigration office here in Chiang Mai, it was shut but a guy outside told me that a 30 day extension on our visas would cost 1900B each.

Wouldnt it be cheaper for us to drive up to Mae Sai, cross the border and come back with a 30 day visa on arrival than pay 5700B for a month extension?

Am I missing something? Does the visa on arrival cost money?

Correct , the 30 days extension cost 1900.-/each.

I assume your Tourist Visa is Single entry.

After you left the country und entered Burma , you can turn back and upon entry you get 30days stamped in .

Cost nothing, but I think you have to pay USD 5.-/each for entry to Burma.

Looks like to cheaper alternative.

You not missing anything................the 30 day extension is a good option for the single traveller who doesnt want the hassle of making a border trip but wants to extend beyond the 60 day tourist stay stamp.

As you rightly point out it is more cost effective when there is a family involved to drive to the border and obtain a 30 day upon arrival stamp (nationality permitting)