I was in the same position about two years ago and, due to the high cost of services like ConstantContact/CampaignMonitor, I voted to build my own. After two years, I had built my list up to about 10k users and my web host finally came to me and asked me to move it off to another app/site/VPS.

It took me about a day to implement, I guess, and I think it saved me probably $2500-$5000 even when I consider the time value of my development time.

I advise that, if you can charge yourself less than $150/hr for your time and you will be sending to a small amount of ppl initially, build it yourself.