Truth in posting here.
I've never been to Siem Reap but I've watched a lot of cost of living, apartment hunting videos about the place.
You need to compare like to like places between Thailand and Cambodia.
Siem Reap is a desirable place to live there.
Perhaps it can be compared somewhat to Chiang Mai in quality of amenities for expats except for the poor health care options there.
So a better question is Siem Reap cheaper for the same level of living than Chiang Mai?
My guestimate is YES, and if you choose cheap accomodations you should be able to save some money. maybe 100 to 400 dollars a month max.
Now if you're willing to living in some bummphuck Isaan backwater, yeah you could go even cheaper.