Would be possible to get a surgery to create a monolid? How much would this cost?

The crease can be made smaller but not completely erased. If the goal is to alter the appearance of one's race, the entire boney orbit and forehead area would need to be changed - it's not just the eye fold that makes gives someone the appearance of a particular ethnicity.

It is impossible to create monolids out of Caucasian eyelids. Once the crease is there it is impossible to get rid of.  

Caucasian eyelids typically have a well formed eyelid crease whereas some Asian eyelids have a minimal or absent lid crease. Making a Caucasian eyelid crease disappear is difficult but can be performed. It would require both dissecting subcutaneous fibers which essentially form the eyelid crease and fat transfer in this plane to ensure the crease does not reform.