Hmm .... depends on condition, and you don't say if it's gas or diesel. A running D14 gas in good condition without a loader seems to bring between $3000 and $3600 these days. I've seen D15 Series I and D15 Series II advertised for more and less than that.
If it's a trip-bucket loader (uses a spring to reset the bucket, not hydraulics) it may add a few hundred to the price. A hydraulic bucket would add a good bit more. Series II is worth more (to me) than a Series I - and has 6 more hp.
Hope that at least gets you started. D15's are available but you don't see them sold regularly. Good tractors.