Really depends on what you deem as 'reasonable' as far as price goes, where you are willing to buy from and how quick you need it.
I've seen ones for $30 on Shapeways (5+ day deliver), $16 on Ebay from China (Gawd only knows how long deliver). You are best off looking in the classifieds on sites like this for a 'spares or repairs' unit if you can wait. If not I fear you'll end up having to bite the DJI bullet.
The other thing to consider is making your own mount without the Anti vibration spheres... I've used my Zenmuse with no Jello on my XuGong without the mount. As long as you have well balanced props and motors it will work well.