I wish they would offer a "Linode 256" VPS for $10 a month, but I doubt that they will. Guess that would make them look cheap, which they're trying to avoid.
The difference is that they sell their boxes with prices proportional to memory so that doubling resources costs double (25.6MB RAM per dollar), whereas at prgmr, you charge a flat $4-per-guest fee and then charge linearly for space on top of that (64 MB per dollar).
Bonus fact: If you both were to offer a 173 MB size, the costs would be almost the same (~$6.70/mo).