I bought my great-uncle's papers a couple of years ago. I didn't learn a great deal that was new, mainly addresses, but still interesting & worth having.
When you have found his name on the National Archive site, you will see a Piece Reference, will will be the papers referring to that person. Click on Order printed or digital copies of the record, & carry on with your order, & you should see that they will send you an estimate for your order, then you can decide if you think it's worth ordering.
I got 16 pages, A3 size, for about £8 including postage, which I thought was good value. Prices will no doubt have gone up since then.
Betty