Purchasing small amount of property from adjacent lot

Hey All, I am hoping someone on here has been in a similar situation and has some knowledge to share. 

One of my properties is roughly 500 SQFT shy of being able to subdivide into a separate lot. I have done my homework and a few of the neighboring properties have spare sqft they can sell and still be within the size to subdivide if they wish. Really I am just looking for knowledge on what it may cost to buy such a small amount of land. 

I have gotten a new survey on my property, my realtor has reached out to the property’s selling agent (the lot is up for sale), getting in touch with my town’s zoning department has been a hassle. 

Thank you in advance!