There are now quite a few really good detectors priced around the $500 mark.
(prices are recommended retail in Aus)
Minelab Vanquish 340 - $399
Nokta Makro Simplex+ $475
Minelab Vanquish 440 $499
Quest Q30 $569
Minelab Vanquish 540 $599
Nokta Makro Simplex+ WHP $629
Having a quick look on youtube, there are many comparison videos with varied content.
Here's some videos I've come across:
(has english subtitles)
This one's very very long and no subtitles, but good the hear the different audio between the units.
Vanquish 540 vs Simplex WHP -
(US prices, but a good comparison on the features)
Simplex vs Vanq 540, sensitivity to small targets
There's a tonne more, and often some good info to weed out in the comments.