Depending on the reader it may be cheapest to pay the $150. The link that rage posted works with a lot of older systems but anything like a HID iClass reader it won't be able to emulate the signal. I had to eat $5k on my last building access system job because I thought I could get away with generic non-proprietary hardware.
If the hardware is proprietary it won't work. If you can find out how long they have had the system you can have an idea if it's likely to work around (most likely pre-2007 you should be ok).
TLDR: For $20.00 it's worth a shot but if it's a newer system probably better off spending the 150 up front.
2017 VW Jetta 1.4T (winter car)
2010 F-350 Nortruck Tool Shed
2007 BMW 5 (Wifey's)
2004 Audi A4 1.8T 6spd Stage III w/ Meth
1985 F-250 6.9L IDI w/ATS turbo