This floorplan was going for $575 SF in 2008. Current listings are averaging $554SF. 20256 Northbrook SQ is at $533. Depending on upgrades the current list price is reasonable. and a good starting point.
20325 Northglen SQ is under contract an scheduled to close at the end of April. The list price was $599K. It was updated nicely with Andersen windows, granite counters over cherry cabinets and stainless steel appliances.
Good luck on your offer. Feel free to email or call if you need assistance.