I've noticed that the Rick Steves Tour single supplement seems to be very low and I'm wondering how it can be so reasonable (by way of comparison, I've been on several cruises where the single price is basically double the 'per person double occupancy' rate; so the 'supplement' is basically 100% of base price).
Consider two people traveling together, vs two people traveling separately and paying the single supplement on an RS Tour. A typical Rick Steves hotel room (shared by 2) costs about $200-$300 (based on our experience of staying at those same hotels as a non-tour guest - the actual number isn't too important in the discussion here). If two people travel separately, with 2 rooms, then that would cost about $400-$600. Now, single rooms are typically the smallest and cheapest the hotel has to offer, so less expensive, but I doubt they are that much cheaper.
The Single Supplement on a tour we are looking at is $500 on a 10 day tour, so $50/day. How can Rick Steves procure an extra room for just $50 more?