Diamond matt is more expensive but thats a bit steep. Haven't bought it for a long time as i don't like it but my guess would be around £50-60 at a good decorators merchant.
I got some free from Dulux for a claim against their white gloss yellowing. Obviously I didn't pay for it but they generated an invoice to themselves for 2x 5L tins Diamond Matt and 2x 2.5L tins of water based gloss. The invoice was about £150 IIRC (about 2 years ago so I can't remember exactly), so I'd go with dcdec's suggestion of about £50-60 per 5L.
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