It get even more exciting when you consider that $110M is the cost to NASA. The cost to SpaceX could quickly reach an order of magnitude lower as it becomes more frequent and reused. As a bonus, the increased profits pay for the next ship, dropping costs even further.

An order of magnitude is a stretch, I think. But a 25% savings is probably pretty doable, which drops the cost to $82.5M. Consider that Richard Garriott allegedly paid $30M for his trip to the ISS, so if you have three passengers and one crew, you're turning a profit of $7.5M

Nope, way less than that. A single expendable rocket may have that margin, although I'd expect even better. Accounting for 5 flight of a si glen booster and capsule, that cost drops precipitously to at least 60-70% percent savings.

They're also only using two of the four seats. And that's on top of taking out seats they originally had planned (maybe they could put them back in at some point?)

From what I have read NASA has opted for the 4 seat version only, and then placed cargo under them where the other 3 seats would be. SpaceX has designed the capsule to carry 7 if you want an all passenger config but it will need to be heading to a destination fairly quickly as it doesn't leave much space left over. That said, even at 7 it looks like it would still be roomier than a Soyuz at 3 people.

does anyone know why it took so long for us to develop resuable rocket? Isn't reusable rocket the natural next step for space launching capabilities?

Propulsive landing is really hard, see the failure montage from Spacex as they iteratively worked on it. SpaceX also had a very strong incentive to develop the tech, since they are a commercial launch provider and could make more money. All the engineering was certainly not free, and the vehicle has to make some compromises in terms of lift capacity and such to make it work. Propulsive landing only looks easy now, but it is a supreme feat of engineering.