In the NW, the majority of the MAs are in the £3500/£4500 mark: the government has given universities 10k bursaries to use this year, and our students are doing very well out of them - some of them still work part-time, but others are just cutting their cloth (average student rent etc between £4 and 500 for somewhere close to the department and fancy: lower if they are prepared to use a bus and don't want en suite etc)
Courses with masses of foreign students on have far higher fees - they are buying the access to London, often, and the name of the university on their CV: it doesn't really equate to the quality of the course itself, in many cases. Place like Oxford charge top whack because they can - simple supply and demand. It's well worth students really doing their research on where they want to go - and also, whether it's worth taking on the extra debt. Don't forget too that many places offer merit-related funding, usually of a fee waiver but sometimes also a stipend too - and some also have bursaries based on WP etc Some institutions also offer fee discounts for their own graduates.
Your MA is far more about the course itself, rather than the institution (unlike undergrad), in terms of "worth" - and if your DC want to go into academia, it's probably better to apply for a 1+3 (which is intensely competitive) and go to university where there is a DTC, to improve their chances of +3 funding: institutions that have a DTC favour their own first when it comes to applications.