Is it expensive to set up a wi-fi situation, since only commercial/bigger buildings and the like have it ?

Set-up a wifi situation? I have a wifi situation in my home....I bought a wifi router/modem, connected my phone line/dsl connection to it, and waa laa I have my relatively lowcost wifi situation.

Otherwise, sign-up for a plan with TrueMove/Wifi.....or, seek out free spots like the new McCafe's.

Is it expensive to set up a wi-fi situation, since only commercial/bigger buildings and the like have it ?

Very inexpensive today. Stop by any place that sells computer accessories. They can show you what they have.

I have several routers at home. It is that inexpensive.

Is it expensive to set up a wi-fi situation, since only commercial/bigger buildings and the like have it ?

Well, it really depends exactly what you want to do, a few pointers :-

1. If you're going to offer this to clients you'll need a "commercial" ADSL package otherwise you may run foul of the terms of your ISP
   
2. The range of the Wi-Fi signal is short in buildings with lots of concrete, you'll certainly need at least one access point per floor and a proper wireless survey performed (particularly if you're going to charge for it, clients don't like paying for iffy connections)
   
3. If you intend controlling access you'll need some form of access server
   
4. You've got to run cables to all the access points (using wireless repeaters is a non-starter)
   

In short, yes, it CAN get quite costly. But if you've got a small pub and you want to cover just the bar, cheap as chips