Welcome aboard.
To be totally honest, I don't think there's anything that's going to cost under $500 without it being a Bruder-type conversion.
The obvious solutions for that price range are only toy-quality units available at the local mall and Target/Wal-Mart stores that may supply them.
With that said, don't lose hope... We have a great group of people here that have "mad skills" and can guide you in the art of master building.
Read the posted materials and ask questions.
Let this be an inspiration for you...
Toy dozer has RC potential!
- RCT