What country are you in?
Cheapest wired solution with a USB interface is probably a parallel port to USB converter. These are sold for as little as $US4 each (links at end).
For up to 8 channels (or even a few more with care) almost no other hardware is needed.
Depending on the hardware in the adapter that you use you may not need the resistors shown here.
Worst case you need one resistor per switch
Depending on the adapter, you may find that the inputs are pulled high or low by default.
If they are pulled low then you may be able to pull them high with a switch.
If they are pulled high then you may be able to pull them low with a switch.
If they are inconsistent (neither high or low reliably) then using resistors as shown will allow you to pull them to the other polarity with a switch.
Note that in the diagram the resistor is shown going to v+ and the switch to ground, but these may be swapped if desired.

This diagram comes from the excellent tutorial referred to below.
Here is a Parallel port switch input tutorial. This is for a traditional parallel port but much is applicable.
Another tutorial. Not so useful, but shows connections
Here are a few example images of the sort of device I mean using Yahoo search. Many more available.
Example only:

Here are some examples for sale These are from $7.99 each but you can probably find cheaper.
Here are some from $4!