I got my 2009 Mac Pro for $150 CDN about five years ago. I just noticed you’re in Thailand so the local market may very well be different over there and maybe this ends up as a decent price?
What are you going to do with it? The latest OSX you can use is El Capitan (I think) which doesn't receive security updates anymore since 2018. I wouldn't feel safe doing banking or anything with sensitive info/data. In my opinion this Mac Pro isn't suited for professional stuff anymore and too outdated for regular use.
Not sure what electricity prices are over there but this definitely chews a nice ton. a 404 on single core is less than half of a consumer/non-pro i5 from 2011
What other options are available? Maxing out is 10.11.6 is also going to be a problem for app support. Technically, you could patch it but if you are willing to do that, you might as well build a newer hackintosh to run the greatest and latest.