Too little information to say with any accuracy, as its value depends on condition ("good condition" doesn't really say enough - just being dirty inside for example will drastically affect what most people will pay), and whether it's a known model by a professional conversion company.
Although Ebay doesn't tell the complete story, looking at comparable vans under their "completed listings" category will give an idea, provided you ignore the scammers and the dealers.
I'd guess at a value somewhere between £1.5k for something rough to maybe £6k for an excellent example by a known company.
The problem is, some people's "Good condition" is anothers 'shed'.
Ive converted quite a few T4's and would suggest something between £2K and £3K but would also suggest the very lower end as its only a 1.9and its 20 years old. It will also depend on what the 'Good condition' is and whether its a home conversion or factory jobbie.
Get one of the VW magazines for some information. T4s are becoming collectable if it's in good condition, or it may be of interest to someone who can restore if not.