If you value your miles at a penny apiece at the moment they enter your FF account, your breakeven point for comparison against iDine Prime is $500 spent per year.

If you value your miles at 1.5 cents apiece on the day they enter your account, then you need to spend $1000 per year in iDine to make it worthwhile to switch.

I tend to value my miles at the time I use them as 1.5 cents apiece - maybe a bit less. Since my miles tend to sit there for a while, I think of them as about a penny apiece when I'm comparing them to liquid cash. (One could argue that they are worth even less when traded for cash, but I allow myself 1 penny because this is kind of a hobby. )

The wildcard for me right now is Lifetime Gold on AA. I'm at 900+ lifetime on AA, so I'm channeling all mileage-earning energies to them right now. Once I cross a million, I'm switching to iDine Prime. I spend about $75/month in iDine, so it's really a no brainer once I'm earning "normal" miles as opposed to these "really special almost-to-one-million" AA miles.