So it boils down to burning an extra 15,000 miles or paying 25,000 THB. That's a no brainer for me, I'd burn the miles. In terms of USD, that burn works out to about 4.4 cents a mile. An excellent deal IMHO. For my situation, I like to figure that miles are worth somewhere between 1 and 2 cents per mile. I don't know what's the average selling price of miles across all airlines, but most I've seen are at or slightly above the 2 cent mark, which IMHO is a bit high compared to what value I receive for them when I burn them as I usually shop around before buying a ticket and get pretty good deals. If your used to paying a lot for your tickets, then the burn value would be higher for you than for me.

Just one question, are you sure about only needing to spend 15,000 more miles for business compared to economy? That sounds awfully cheap to me. My experience has been that typically you need around double the miles for business class compared to economy. Maybe there's some special promotion on now?

Edit: I just had a look at the upgrade chart and it looks like depending on what kind of an economy ticket you're buying, you'll need either 24,000 or 30,000 miles to upgrade her to business class, not 60,000. That means your burning either 36,000 or 30,000 miles extra in exchange for spending 25,000 THB, so that works out to or 1.86 cents per mile or 2.22 cents per mile. At 1.86 cents per mile (24,000 mile upgrade) I'd say it's a toss-up for me. At 2.22 cents per mile (30,000 mile upgrade), I'd say go for it. But you really need to calculate what the miles are worth to you in your situation before making a decision which way is best for you.