Below 25,000 miles, that makes the price just about 3 cents a piece. That’s high, and much higher than I would ever pay for a United mile unless I needed just a few more to redeem for an award. I would recommend the same for you.
As you can see from the list of prices on the right, it’s actually cheaper to buy 25,000 miles than 24,000. That price is 2.44 cents per mile. More reasonable, and if you were considering buying some miles for an award, you might be better off buying 25,000 and using the rest later.
To put these prices in context, you’ll usually pay just over 3 cents a piece via United’s Award Accelerator for an existing reservation. So, both of these prices are lower than that, but still not what I would call a great deal unless you had a specific purpose in mind where the math made sense.