I recently booked a trip to Asia with lap infants. This is on the confirmation emails:

Booked on ual.com
Adult Base fare: 1,354.60USD
Tax/Fee/Charge: 71.40 USD
Total: 1,426.00 USD per adult

Called up the 1k desk to add lap infant:
Base fare: 114.00 USD
Tax/Fee/Charge: 32.60 USD
20.00 XT (Federal excise taxes)
Total for Infant: $166.60 USD

Also FYI: I subsequently upgraded to C by calling the 1K desk. Upgrades were confirmed and no mention of any incremental cost to the infants as some folks have reported.