I don't know for sure, but from what I've seen it's the easiest and cheapest system to use. A lot of people who set up their own ICANN accredited registrars use it too.
When I first started using it I found it a hard adjustment. Even now, I think it's a bit clunky, but I've gotten used to it.
The fact of the matter is that its the most comprehensive one out there..and there isnt any big competition to it cos it was one of the first of its kind..if you want to get accredited by ICANN, you need your own system for making calls to every registry..they basically have a strict policy in place to check if you are technically capable or not..and logicboxes just makes it easy by providing a ready made system..I dun like the way it looks..takes me back a couple of years
I had spoken to someone who had a registrar account with them..their fee is indeed on per domain basis..I don't exactly know the exact charge per domain..but it was a couple of cents on every domains cost..plus they had a one time consultancy fee and that ranged from person to person..depending upon how easy it is for them to get him accredited..