I recall that had a fairly impressive marketing video on their page previously. This could have cost $50k alone.
I think the main unexpected cost that you can incur is advertising/marketing. You can build the software cheaply, but it's very hard to get the word out without spending money (and it can add up very fast). Considering their fast growth, I would also assume significant spend in this area.
As long as you are getting a positive ROI from this spend, then it's worth throwing as much as you can at it, but obviously you need to have it to throw... that may be one the keys to their success.
I think the main unexpected cost that you can incur is advertising/marketing. You can build the software cheaply, but it's very hard to get the word out without spending money (and it can add up very fast). Considering their fast growth, I would also assume significant spend in this area.
As long as you are getting a positive ROI from this spend, then it's worth throwing as much as you can at it, but obviously you need to have it to throw... that may be one the keys to their success.