Mackay tartans can be a bit confusing as to which ones you are talking about. There is MacKay Modern, MacKay (aka Morgan ancient) and MacKay blue, any of which may be what you are looking for, many of which are interchangeably called one another. More specific details and maybe a picture of your actual desired tartan would be helpful.

For MacKay Modern Scotweb's tartan search engine turns up three 16 oz tartans by Lochcarron Strome, Marton Mills Jura, and House of Edgar Nevis range, all at different price points. With your xmarks discount (obtained by entering their website via clicking on their advert top right this page) you can get Scotweb (an advertiser here) to make your 8 yd Balmoral knifepleated partially handsewn kilt in MM Jura for $332.52US or in HoE Nevis for $303.47 including shipping. Lochcarron Strome would be considerably more. It may be able to be had cheaper at other non-advertising vendors but not by much. Compare to Rocky's or Steve's prices or those of the other vendors on the site.
You may be able to talk Josh at Skye Highland Outfitters into doing the MacKay modern in his Standard Kilt for $299 but it does not list Mackay modern as a tartan option, only Mackay Ancient, Mackay Ancient Blue, and Mackay Blue, which may or may not be what you are desiring in tartan coloration. Heritage of Scotland, not an advertiser here and often not recommended by forum members for a variety of reasons, lists their 8 yd machine sewn kilt in MacKay Modern at $260 for MM Jura and $270 for HoE Nevis including shipping. I have several of their kilts and they are well made, but if you search the forum archives you can get more history on the mixed emotions about potentially ordering from them and why. I have never had a problem.

My personal recommendation would be use Scotweb for their Balmoral 8yd MacKay Modern in HoE Nevis for just a tad over $300---good service, quality workmanship, Nick their proprietor is a regular here on the forum and will ferret out problems if they come up, and turnaround time is usually pretty good, if the tartan is in stock. If you can come up with a better price on comparable stuff from another xmarks vendor then you could go that way and save a few bucks along the way.