Prices for non-Belgian invited speakers, in GBP, based on where they are located to the best of my knowledge:
Dr. Stephen Coles (LAX) 939
Dr. Aubrey de Grey (LHR) 126
Paul McGlothin (EWR) 728
Dr. Michael Rose (LAX) 939
Danila Medvedev (LED) 212
Valerija Pride (LED) 212
Stephen Spindler (LAX) 939
Leonid Gavrilov (ORD) 806
David Styles (MAN) 97
Sebastian Sethe (LBA) 152
Total in GBP: 5150
Approx. in USD: 7881
N.B: Of Danila and Valerija, chances are only one will speak, bearing in mind either could easily speak for the other.
N.B.Q: Airfare prices fluctuate by the minute, so these prices are subject to shifting somewhat.