On anything over 10 million I'll try to split 50/50 between upfront and monthly with months varying from 6 to 48 depending on how much I'm willing to pay per month, I try to avoid spending more than 2 million a month for a player.
I try to sell for up front, but have noticed I get a lot more offers and significantly higher fees if I offer a player for a couple million up front and another 10-20 million over monthly installments.
Also I always sell young players with potential with 35%+ sell on fee, and buy players without a sell on fee.