I will keep that 500$ with me. And will promote the website with my own ways. I never used money for promoting or programming or coding stuffs,as i do all that myself.
Depends on the the type of website. I'd try to get the website to show on tv or make a entertaining youtube video about it or something.
Do not forget that website is type of investment. If you keep $500 you will have $500-inflation in 6 mnths time, but if you invest it to website you can have $750 in 6 mnths. About doing all yourself... Aguable point. I bring you only 2 teasons: 1. Time which you spend on costs money 2. It takes longer to promote webs if you have even 10 webs
$100 for some good links in the start, and then the $400 in just making writers write some good number of articles for your site, as content always rocks