The 9" Ford one is certainly around that price from what I've heard.

There was an interesting thread that Porkie started a couple of weeks back about diff options for the 7" rear end that you might find interesting.
I think he decided to get a plate type diff in the end.
There were approximate costs for the different options contained in the thread.
If I understand correctly you need to have some additional work done to fit a Quaife type.

A lot of the big power cars seem to either run a 9" viscous or a plate type diff.
Depends really on what sort of driving you do.
People seem to suggest a smaller viscous one like a 7" will wilt quickly under heat build-up with hard usage if you don't have additional cooling.

Interestingly, Nissan installed factory fit diff coolers on the S13 and S14 200SX road cars - they had viscous limited slip diffs. The cooling was enabled by a speed and temp switch.
I think on the S13 you had to be going over 6mph and have the diff oil over 130 deg C for it to open the stat to the cooler.
On the S14 type the speed factor was increased to 75mph.
Many people seem to bin them as they weren't used in ordinary driving. A few people wired up their cooler switches to a dash light to see if they ever utilised the cooler. A few of the drifters said they only came on after prolonged 'drift' sessions of 20 minutes and 60mph plus slides.
The s14 200SX viscous diff has a higher Nm rating as standard than the 7.5" Ford one. Approx 30Nm versus 24Nm. My understanding is that the higher you bump up the Nm rating on a viscous type, the more aggressive the "locking" will be but by the same token the more front end push you will create ( as with a welded diff ).

If you buy a secondhand 7" or 7.5" Ford one you will find that many of them don't even have the original level of lock as the slicon fluid within gets 'tired' over the years