First of all, I'd like to clear the position of the completion rate. As you yourself are a freelancer, you do know that completion rate here is a two way street. Like if I even complete the work in time but the client never responds back, the completion rate will drop. I hope you'd understand that we may need to consider the experience rather than the number presented by the website.
Regarding the project, I have checked robinhood.com. Here is the response to your questions.
Time:
A website of that level can be converted to WordPress in 5 days on an avg.
Cost:
If we are talking about a one time development, such website can cost upto $300USD. But, if we are talking about a long run work, I can reduce it to $200USD.
Number of Projects:
I can deliver minimum 10 websites just on my own. I know that doesn't go with the math of 5 days per website and 10 websites per month. But I do know how to work on multiple websites of such extent. So, that is doable. If we get a more orders, we can obviously do arrangements accordingly.
Please do let me know what do you think.