I entered to Thailand on the 26th December and received 30 days as usual. My flight back is on the 23rd January..

I went to Laos for a 1 week trip as planned and when I came back I didn't realise my 30 days will be downgraded to 15 days as I entered via land border.

Now my 15 days will expire on the 17th January when my flight back is on the 23rd so I'll be 6 days short.

What would be my best option? I am on a slightly low budget.

Can I get a 7 day extension from Bangkok Immigration and how much would this cost?

You were on a visa exempt entry (not a tourist visa), and received a new visa exempt entry when coming back from Laos. As you found out visa exempt entries via land borders only gives you 15 days (unless you are from a G7 country). Your current 15 day permission to stay can be extended for 30 days at Bangkok Immigration for a fee of 1,900 baht.

Sophon