I made some progress by adding:
$query->set( 'meta_type', 'decimal' );
But when i fill in 10 as minprice and 20 as maxprice I also get results like 9,86 and 20,30.
When i fill in 10.50 as minprice and 20 as maxprice, everything before 10,50 is gone as it shoot, but 20,30 is still there.