Depending on how long the house has been on the market and how desperate the sellers they may not budge much at all. If you aren't dead set then by all means go for lower offers to see how low you can go but do note that the sellers might simply get annoyed , say no and ignore any other offer (or only accept the 250 asking price), in which case do something like 232 and get the agent to explain that any more would be impossible due to mortgage etc.. If you really do want the house just be cautious about low balling, 240k would be a fair price, and don't forget house can and do sell for more than the advertised price.
Pricing relative to neighbors can be difficult. Factors will not only include size, but room layouts, number of bedrooms, garage, Garden size, is the garden nicely landscaped, quality of the finish, exposure. Also just depends, if the neighbors property were sold due to bank foreclosure or desperation by the owners then obviously values will be less.