Before start testing the circuit you need to consider there is a small loss due to the diode losses, and additional losses in the 555's transistor switched output, so the output voltage is lower than expected, at 6V power supply this may be around -4V.
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For modular solutions, this is probably the best you can do: **broken link removed**
If you want it in even less than $10USD, then you'll have to make your own circuit.
Also consider the possibility of generating just the +12V rail, then generating the -12V rail from that using an inverting charge pump. At low current levels charge pumps can be very cost effective. You could even generate everything with charge pumps, if you don't mind poorer regulation.