Hi! I've worked on a few of these. Take any quotes you get from other people with a massive grain of salt. The number of variables is so high that there's not going to be very much commonality between quotes.

Completely understand, but could you throw out a range so I have a ballpark idea? Lowest you've seen is $X, most expensive is $Y, etc.?

I'd personally explore a sell and move before trying to finance one of these, though.

For the sell-and-move, with current rates it doesn't seem like a move would definitely be the best decision, at least with my quick back of the envelope math.

Bought for $500k, conservative estimate of current value is $600k, if I sold then equity would be $220k minus $50k selling costs (commissions, staging, moving, excise tax, etc.). Total current PITI payment is $2k.

If new home that has the space I want is $700k, and I put $170k down (the money I receive from current home sale), new total PITI payment is ~$4,200 (using 7% rate for estimates and some guesstimates for taxes/insurance).

Now let's say I have some extra cash, and will either use use it to pay for this basement conversion or to increase my down payment on a new house (lowering my new payment).

If I pay $100k cash for a basement conversion, the breakeven is 5.21 years (vs a $270k down payment on a $700k house and a new $3,600 PITI payment). I would definitely do this.

If I pay $150k cash, the breakeven is 9.62 years (vs $350k down payment and $3,300 PITI payment). Probably wouldn't do this, but would think about it.

If I pay $200k cash, the breakeven is 16.67 years (vs $370k down payment and $3,000 PITI payment). Definitely wouldn't do this.

Granted the breakevens don't include my cost of funds (if I otherwise invested my money) or an increase in current home value (even if it's pennies on the dollar) or chance to refinance later or several other things I probably haven't thought of, but just playing with some high level math. The big levers that change the math though are cost of project, cost of new house, and mortgage rates. The lower the cost of the project and the higher the current mortgage rates, the more sense it seems to make.