On Amazon.com
TL-ANT2415D is 46 dollars. And you also need pigtail cable like TL-Ant24pt to connect the antenna to the router...
TL-WA5110G is only 37 dollars.

Serving 0.5KM will be over-demanding for Tl-WA5110G if you have too many client devices. ..Anyway it's only an old 802.11G router..

If your client devices distribute in the same direction, forget the 5110G with external antenna, buy a TL-WA7210N instead.

Another thing, the client devices should also powerful enough to talk back too the router. So when there are obstacles in the way, you may not get your desired results.