It all depends on how you use them.

25,000 miles could be used for a 600 mile round trip US domestic economy flight that might also be on sale at the same time on Southwest Airlines for US$38 round trip. In this case, I would value them at 15/100 of one cent.

Or 100,000 miles might also be used for a
premium class ticket from the US to some exotic destination in Africa (for example, using 100,000 AA miles to secure a business class seat on CX from JFK-JNB) that would normally cost, say, US$12,500.00. In this case, I would value the miles at 12.5 cents per mile.

These are two extreme examples, but generally speaking I find the best value in using miles is for premium class international tickets.