Retrotec RDF-900 for $500?

Just a homeowner who recently moved into a leaky house and also building a garage/shop next door. Would very much like to have the benefit of blower door testing, but over the course of time would be quite cost prohibitive to have tests done 5-8 times over the course of a couple years.

A Retrotec RDF-900 on eBay for $500 looks to be in good condition from what I can see, Im wondering what you guys thought of the price for that equipment, or what else can I do to somehow measure the airtightness of these buildings as I go forward trying to seal them up.

Thanks, Rob