I like to use all or some of the following
1) Get some press on niche blog / news outlet / industry publication
2) Leverage my personal network
3) Facebook Ads - cheap CPC but not as qualified
4) Sponsor a podcast that's relevant with big audience
5) Create great content and attract the users for free

Those are the top 5, everything else is just another paid channel.

That being said, Twitter Ads are cheap and they have a new lead gen tool that could work for you = more cost effective than Adwords.