The Irish county tartans, including the County Mayo, are woven by the House of Edgar, in Scotland, in a 13 oz (medium weight) worsted wool. It is really good kilting cloth, and I really like their colors. I think you'll be satisfied with the product.

The price that company is offering for their 4 yard casual kilt seems fine. When you adjust for the poud to dollar conversion, it's about the same price (well, a little more, but in the same ball park) as we offer the same product for.
http://www.scottishtartans.org/Irish.htm

So, seems to be a fair price for a good product.

BTW, if you have't yet seen your Mayo tartan, follow the link above and you can see all the Irish county tartans.

Aye,
Matt