I looked at several airlines and they would generally sell you 7,500-40,000 miles at 2-3 cents per mile so that is a value that won't cost you your account.

The value that I have used them for has been 40K miles for a $1600 ticket or 4 cpm and 160K miles for a 20 stop round the world ticket in coach. That ticket was probably worth $8-10K so about 5-6 cpm, but like others have said I would not have bought the ticket for that price.

My next ticket will likely be SEA-NBO in British first for 160K miles. The list on that ticket is about $16,000. Coach class is about $2500. If I had to buy this ticket I would probably sit in the back.

I think overall I value miles at about 4 cpm for redemtion, but try to aquire them at 2 cpm including bonuses.