So 4 STBs in total? Yeah seems about right to me.
Worth noting - if you were on Sky each of those extra boxes would cost you a tenner, so with £24 for all the basic packs (equivalent to VM XL TV), then £30 extra for the boxes, £13 for the line rental = = £67
before you think about what Broadband you might need (and Sky don't have an effective competitor to the 50mbps service)
£82 is the correct price according to the VM website. (£75.49 is the price with 2 extra STBs, so an extra 1 at £6.50 = £81.99).
http://shop.virginmedia.com/build-your-bundle if you're in doubt.
Of course, a call to customer services never wouldn't do any harm (so long as you're out of contract), but don't expect miracles.