From my experience, mentorship can provide structure, accountability, and networking opportunities, but it often comes at a high cost.

What costs/fees should I look out for?

Finding durable products at reasonable costs. 

The 2-3% points in extra cost are worth it if you plan on using as I described above because the use would be for a few  months at most and due to that short term use you can survive a rate adjustment up.But, if you have no plan to payoff the debt like when using for a down payment on a long term hold why pay the extra cost for flexibility and have the additional risk of the adjustable rate with the amortization looming when you can get a fixed rate second mortgage. 

They have cost my business tens of thousands of dollars and countless wasted hours of follow ups and missed opportunities.

Those situations go bad more often than not, costing you money and the friendship.

Even if I were found guilty, the cost would be in the thousands, not in the millions.

Is just the fact that no garage could be a no go for some buyers enough to build it even if it only raises the value by the actual cost?

It is going to cost more than you expect, and you'll have that "Oh, crap!

Renovations can cost more than you expect, add 10% to your overall number.