I have an opportunity to buy a used Bernina 440qe with BSR and embroidery unit.
It's 4 years old and hasn't been used much. I know the seller and I will take the unit to have it serviced before the purchase is final (just in case).
what would be a reasonable price to pay?

-April

I would say one-half of the purchase price or less. Have seen this done before on higher-end, newer machines that have not gotten a lot of usage.

After I posted, I went to eBay and checked on "completed listings" for this model. The ones I saw that actually sold ranged in price from $2,000 to $2,600. (I did not go into individual listings to see how they would compare in detail to the machine you are looking at. You might want to do that. I think the higher end prices were the ones with the BSR and foot.)

Check ebay.

I have this machine and I love love love it!!!!! Did I mention I "love it". I would recommend it if you could get a good price.

I also have this machine and I love it. Just got moved into my house 6 weeks ago after a year in 5th wheel. Just got to the embroidery part and I am hooked. Not buying fabric right now as I am investing in thread LOL. I say get it if you can. Those ebay prices seem reasonable.

Thank you to all that responded. I appreciate you all taking the time to offer some guidance.
-April