I would ask the mechanic to confirm the shock was leaking, and to make sure it was not a leak from something else (hose, accumulator, fitting, leaky axle boot spraying oil, etc) making it look like a shock leaking. From what I have read here on the forum, the shocks often are the last thing to leak in the system, as they are pretty heavy duty. If it is the shock for sure, I am sure that you can get a good used one here in the classifieds or from a junkyard for much less than $500.
That being said, I have had my '85 for just over a year, and I have replaced the accumulators (very easy), rebuilt the control valve (easy), replaced the filter in the fluid reservoir (very easy), and flushed the system with new fluid (super easy). All in all, under $350 in parts, easy to do the work (even with basic tools), and I've had no problems so far.