A new promotion (until 30 April) offers 8 miles per $1 on purchases at Expedia (min $2,500 in a single transaction, otherwise 6 miles).I don’t really use OTAs to book hotels (Because you can’t earn credit for elite status when you do so) but I’ll happily use it for airline tickets.
Is it difficult to get $2,500 in a single transaction? Yes and no. If I book a long-haul premium economy ticket with SQ I can just about make that threshold (EDIT: Or, as someone smarter than me pointed out, buying 2 economy class return tickets should just about do it. Doh). 20,000 miles for 1 transaction seems plenty sweet. Remember, unlike hotels, OTA bookings for flight tickets still earn you regular credit with the airlines.