It seems that filtering by decimal field doesn't work at all:
I think I can try to fix it, if it's a confirmed bug.
This behavior is confirmed on rails_admin 0.0.5. When using 0.2.0, it's partially fixed: filter by value 10.0 works OK, but filter by value 10 doesn't affect results.
Update: the real problem with 0.0.5 was in field type. In 0.0.5 it was float, while in 0.2.0 it was decimal. Plus, the stuff with 10.0 and 10 was caused by a condition in build_statement method.
All of mentioned above was fixed in #1393