It is unlikely that you would be successful in forcing contribution from your neighbor for the cost of erecting the original fence. However, you may be successful in refusing to allow the neighbor to attach his or her fence to yours. There will be a lot of facts that will go into this litigation such as a current survey to determine where the fences are located in relation to the property line, etc. Since the value of your claim is $450 I recommend bringing this matter in Small Claims Court as getting an attorney involved at $300/hr is not viable. Good Luck!