You can buy incredibly cheap PC oscilloscopes (i.e. ones that run from a laptop usb port and use the laptop for the display and controls) from China/Hong Kong on eBay. For example, this one http://www.ebay.co.uk/itm/2-Channel-PC-Computer-USB-Digital-Storage-Oscilloscope-/290550560370?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item43a629b672 is only GBP21 including carriage (but only spec'd up to 3kHz), while this one http://www.ebay.co.uk/itm/DSO-2090-100Msa-s-USB-PC-Virtual-Storage-Oscilloscope-/320467863941?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item4a9d5f8985 is spec'd up to 40MHz and costs GBP109. I haven't any experience of these so I don't know if they are any good, but I'm tempted to buy one.
There's even an Arduinoscope sketch to turn an Arduino into a low-frequency PC oscillosope.