It cost £306 for 50 shirts. The more you buy, the cheaper each individual shirt will be. We can sell the shirts for a reasonable price at gigs, and still get some money back with profits going into the band coffers.
Having done t-shirts before for another band, like you say, the larger sizes sell better, so I went for 10 Medium, 20 Large & 20 XLarge. All of the Mediums have been sold, I think we're down to 10 Large and about 12 XL's.
The band had been talking about getting merch / doing a CD of the album, but that's as far as it ever went. I got a tad frustrated with this and just ordered them. I did have to chase people to get the artwork in a suitable resolution to order the shirts, but thankfully, everyone chipped in, so I managed to get 3 quarters of my outlay back, which helped.