Nope, I didn't spend a cent on getting followers. What I spent was time.
Here's how you can get 10,000 more Twitter followers the way I did.
Go to justunfollow.com and sign in with Twitter. Turn off all automatic tweets it wants to send on your behalf (including auto-DMs). It was seeing these automatic tweets and DMs that kept me away from that site for months, but when I found out I could turn them off, I started using the site.
I'm not endorsing justunfollowme.com, there may be better sites out there. It's just what I used.
The first thing you'll see when you get there is a list of tweeters who you follow who don't follow you back. You can, with a free account, unfollow 25 once every 24 hours. If you allow the website to tweet that you are doing that, it's 50. I did that once and got 50 from then on. I don't know if I got lucky or if that's how it's supposed to work. You simply push the blue "Tweet" button to tweet it:
But you don't want to unfollow, not just yet. First you want to follow as many tweeters as the site allows (again, default is 25, if you tweet, 50).
Click on "Keyword Follow (Grow)" in the left had menu. Now, put in a hashtag that fits your genre(s). I used #urbanfantasy, #scifi, and #sciencefiction. Follow most everyone who uses those hastags. You can also use #amwritng and #amreading. Don't follow people with a huge mount of followers but hardly following anyone else because they are very unlikely to follow you.
(I tried #UF for urban fantasy but got a lot of people talking about the University of Florida. I tried #fantasy but got all sorts of things, including people looking for casual sex.)
Follow 25 or 50 tweeters (as many as the site will let you).
Then, sorting by "oldest first" (the default) unfollow 25 or 50 tweeters who don't follow you.
Do this every day (you can do 25 or 50 once every 24 hours) or at least every day you can.
Eventually you'll get down to 100 people who aren't following you back that you follow. When that happened I changed tactics slightly. I still followed 50 people but I only unfollowed until I had 100 (exactly) non-followers. I didn't unfollow anyone who hadn't tweeted since I last had done a batch of following (usually a day) unless they hadn't tweeted for 12 weeks figuring they were inactive at that point. This way, the people I just followed wouldn't be unfollowed because there were at the bottom of the list. Don't update the list or you it'll add in the 50 (or 25) you just followed.
Do this every day or nearly everyday for a year and you should gain around 10,000 followers. After I hit 10,000 I stopped doing this because I apparently hit some critical mass and I get quite a few followers every day (over 25, usually). I follow back the ones that have to do with writing/editing etc. I don't follow ones who are obviously bots.
Was it worth it? I don't know. I do get a heck of a lot of re-tweets (but how many of those are automatic, I don't know but I bet a lot).
But that's how I got 10,000 Twitter followers in just over a year.