Jeffrey Warren has extensively investigated using 555 circuits to trigger cameras via CHDK. This appears to be the least expensive option, his notes are here. for completely DIY 555 circuits, see Mathew Lippincott's note here.

Chris Fastie has explored two other options providing greater degrees of control:

1) using a tiny remote control purchased on ebay. his notes and KAP forum discussions here.

2) using an AuRiCo, or AutoRigController, sold in the states by Brooks Leffler. His notes are here and here. The AuRiCo is quite capable and very small and lightweight, but costs a lot (it can also control two servos). The one sold at Brooxes for US$64 is a special version that DOES NOT HAVE an output for USB shutter release via CHDK. So it requires a US$44 cable (same page) to work via USB. Peter Engels sells a version that does have an output for USB triggering via CHDK.