I have setup a personal account to look at how Outsystems development model works and its capabilities.
So far looks quite interesting and fun.

The SilkUI forum talks of using up SU or software units and running out of licenced SU's if using silk.

For an enterprise account :
How are these calculated/used ?
Purchased in blocks of how many at what cost per year/month ?
Different SU "costs"  for self hosting / hosted / mixed environments ?
SU space used up by each Forge pluggin loaded into my space ?
SU's used per App published or by number of users per app ?
Single Tenant vs Multi-Tenant space costs ?

Anyone able to clarify ?
I have heard at least one person say they saw the Silk UI take up 12K+ SUs.  For pricing, you will need to get a quote by calling Outsystems (http://www.outsystems.com/company/contact-us/) and give them the general specs you need.  In the enterprise version, you can find out how many SUs each component is taking up in the Service Center.  Even though SilkUI themes are awesome, London is still relevant for modern web development and I've found it customizable to my needs.

My recommendation is to focus on what you need for your envisioned product, see if it is financially feasible without having to say "oh, that's too many SUs", and make a decision from there.  If you decide to use Outsystems, I would personally recommend their hosted solution over self hosting and mixed environments since that takes a load off of your back in terms of server responsibility.