Running a Moodle VLE can cost anywhere between £50 & £1000.
In all honesty, it's not the cost of the moodle that is the issue, it's the training & development of staff where cost issues arise.
As far as the platform itself goes, there isn't that much between uniservity & moodle, and even moodle 2 doesn't look amazing and new (unless you pay someone to develop skins for you).
The question you should ask is what are you looking to achieve? Why do you want to replace one vle with another? Are you looking for eportfolios for pupils? Do you want areas for staff to share resources? When you know what you want, then you can look at the technology to support it.
I run 120 moodle sites for my local authority. I'm moodle through and through, whilst I am very technically able, I can not run the background operations of a moodle, VLE, so you need someone to support you technically, (thankfully I have someone!) which involves cost. I've seen administrators moodle courses at £1000 a day that won't even touch the sides of what you need to know to keep your moodle running.
I wouldn't bother hosting internally, the idea of buying servers is ridiculous when you can have a server sat in iceland/sweeden/san francisco that someone else manages. Besides, your school internet feed will not cope with a huge demand if your moodle grows.
You are welcome to private message me if you need more info.