From The Spectator...
Speaking terms
What can Boris Johnson earn as an ex-PM on the speaker circuit? Some payments that Theresa May has received:
£160,370 from JP Morgan for two speeches.
£127,200 for one speaking engagement from the Distinguished Speaker Series of Vero Beach, Florida.
£404,800 for six engagements with the Cambridge Speaker Series of Ross, California.
£109,000 for a speech to the Danish Bar and Law Society.
I think I'd just do the speech work, and cut out the bother of being PM!