We did this last year on a SEA-CDG LHR-SEA ticket. The short story is Delta charged me 10% of the economy class ticket + taxes + $100 "surcharge" with the GUC. I got a good deal on my coach fare ($1000) so I thought this was pretty fair.

The long story is, it took a number of calls to get this. I called the Delta several times and each time was quoted a different amount. On a previous call they were going to charge me 10% of a full D1 fare + taxes. My experience with booking lap infants on award tickets and in this case coach + GUC is that no one seems to know what the rules are. On another occasion - a 1-way award business class ticket on VA metal to Australia, i was quoted between $1300 for the lap infant down to $400!

I would absolutely try calling multiple times until you get something which you think is fair. For award tickets they should at least base the lap infant far on a discounted fare (if your mileage payment reflects that). Maybe try to quote my experience above for the coach + GUC?