I'm a fan. I prefer the low mount to the see-under.
I like the ease of removal to use the bird barrel and easy re-installation to use the slug barrel with a scope. I find that they re-install with negligible loss of zero. I believe most or any loss of zero results more from the changing of the barrel than the removal of the scoped bracket.
Drilling and tapping the receiver for bases or using saddle mounts probably results in a stronger installation but, in both cases, you sacrifice the easy change flexibility of the Convert-a-mount. Really, why trade the flexibility for more strength when you already have strong enough?
I don't know what they go for in the US but based on Canadian prices, I would guess somewhere in the neighborhood of $50.00 US.