Old-skool Rockford amps are very reliable 🙂 I have 3 of them and theyre all more than 4 years old. I bought them all second hand. Maybe I'm just lucky.
Ok so if you don't want to buy secondhand then I'm sure you can find something for your budget!
The latest RF stuff is supposed to be a complete redesign (IE not the re-hash of last years models with a different model name) and is more like the old skool amplifiers.
PPI are fantastic also, as MikePanic said.
Look out for RF, PPi, Orion, JBL if you want to spend less etc.
Make sure you get an amp thats suited to your sub's impedance.