Glancing at the pic of the front of the business, it looks like they'd need about 5 vertical feet to cover both sets of stairs. To keep a ramp at appropriate grade (1:12), that may consume quite a lot of space. Unless I don't understand grades, 60-ish feet.
Once you add permitting costs, business downtime while the front of your building is gone, and possibly removing big chunks of that patio to make room, $100k doesn't seem that unreasonable. And that door looks narrow too; that may well need to be widened.
If interior renovations are required (bathroom not large enough?) it's easy for things to get very expensive, very fast. And in a 100 year old building, you quite likely also have lead paint and asbestos. A small asbestos remediation can easily run $13k (source: did one).
And if the business is going to be closed for more than a week, don't forget paying your employees. You obviously don't have to, but if you tell your employees you aren't paying them for 3-12 weeks, they probably won't still be there when you reopen.