Expensive: Wireless LAN / ethernet. If your PLC have TCP/IP connection, you could maybe use a wireless LAN.
Check for instance:
http://www.wlana.org or do a internet search (Wireless + TCP/IP)
Cheap: Radio modems with RS232/RS485 interfaces.
For example:
http://www.satel.fi/bb/home.nsf/pages/Satelline or
http://www.elpro.com.au/405/405_2.htm
Do a search on the web.
An example:
We have an water and wastewater application in a county. It consist of about 80 PLC's (still increasing)
Every PLC are connected to the top system. About 40 PLC's uses private line modems on leased lines and about 40 is connected in a multidrop using radio modem.
The customer wants to use radio modem on the further expansion because he saves a lot money. (The frequency is free, there is no charge is our country)
Because of geography, all stations can't use a radio modem. But with link stations, also difficult station's can be connected. Also protocols without adressing (point to point protocols) can use radio modem, because some of them can decide the link, that is, you decide wich modems that shall communicate.
You can also use GSM-modems if you don't need an online connection. For installations where there are no other solutions, it is possible to connect f.example once a day or on command read/send data and hang-up.
Has been done on application's out in the wilderness or during installation/service.