Good question, well at the moment currency rate wise $150 turns out to be around £89, $400 turns out to be around £238..
If I wasn't a collector I'd probably spend around £200 to £250 for a vacuum cleaner (that's $335 to $415 roughly) on a good quality vacuum cleaner. Id also be prepared to spend lower than £200 if I spotted something that looked or felt like a good quality vacuum.
That's not to say I don't like a bargain - bizarrely I'd like to spend new on a cheap "plasti-vac" rather than consider something second hand.
The reason being is that I don't consider all plasti-vacs as being a non-investment. Some last longer than others but it very much depends on the amount of care and maintenance you choose to put into the machines, as with mostly other appliances.
On the other side of a coin, not many vacuum cleaners are an investment. They may be seen as an investment if they last long, but to use the real word "investment," to me, means recouping the same price I bought the machine at the same price if I was to sell it. Vacuums are worse than cars - grossly over priced and depreciate far too quickly, regardless of whether they are vintage OR modern.
