Just wanting a quick reply.. If i demolish a building... does it cost resources or do i get resources back ??

Deconstructing a building costs resources and you get none back.

thank you... but does it really make sense ? lol
cause wouldnt it be like selling the scarps.. or harvesting all the scrap metal..crystal coming off the building when it goes boom lol oh well.

It makes sense. Think of it as a penalty for building something you didn't really need. Forces you to think hard about your purchases as you near the limit of your planet's available fields.

I'd suggest that you always leave at least 10% of your planet's slots open for purchases later on that you'll sorely need.