As far as cost, right now I would say you would be looking at a range of between $1 billion (the inflation-adjusted costs of Lucas Oil) and $2.4 billion (the estimated cost of the facility the Browns proposed building in Brook Park in August). Note that doesn't include Paycor demo costs.
And again, that's right now. You have to figure it would take a couple years to plan, design, get financing etc., and a couple more years to build, so those costs will rise. I wouldn't expect it to get done for less than $1.5 billion and honestly they'd probably be lucky to hold it below $2 billion.
Given that the Bengals and the county can't settle on how much each side will pay for the $700 million-ish package of upgrades currently on the table, a sum two or three times that amount is a non-starter, imo.