As an author: I think the immediate sell through on a 99 sale is more pronounced than a free run, but the long tail (which is hard to pinpoint) might give more sales of follow on books in the long run. Like others have said, a free book has no urgency, but at some point, might be read and if it's good...well...sales follow, but those are in trickles after a certain point.
I used to like doing free runs, but eventually, I had to stop and consider very carefully because it messed up alsobots so incredibly badly. Mostly, I do 0.99 cent sales in order to keep those alsobots less hosed.
As for pricing books, I try to price books moderately in the first place and it's entirely based on the cost to get to market and labor. I don't write books quickly at all. The Portals duology took 3 years of labor, one year of that labor exclusive to that pair of books. I also don't write endless series as a way to gobble up money. My books tend to be fat and divided only as necessary.
I recently had a 0.99 sale for Portals, the first time since launch, and even without a bookbub (in KU) it did very well and has about an 80% sell through to the second book. Since it's only two books, that's pretty good and gives the reader one heck of a read. People don't seem to balk at paying 4.99 for 500 pages.
As a Reader: I used to love freebies, but honestly, I've got so many hundreds that I'll never read them, so I've mostly stopped with those. I tend to look closer at a book that's 0.99 simply because I don't want to clutter up my TBR with something that has endless series just to pump more money out of me or a name with no substance. If the author looks like a mill, then I avoid them.
I don't balk at a price for a follow on book if it was that good. I read a lot and do like to buy responsibly, but it's so hard to be really captured by a great read that when I find one, I just buy.
For Tradpub, I'll often get it from Hoopla or Overdrive, including the audio, but in the end, if it's super duper amazing, I'll buy those forms of it to so I can enjoy it again and again. I must have listened to The Year of the Flood in my car during my DC commute a dozen times. So beautiful.
For the Cost Conscious: I've always done a launch price, and I think a lot more authors are doing it. Many readers have a budget for books, and I'm for the reader, so I launch books with a 0.99 price for a few days so that my regular readers can get it cheap. They deserve that! I wish my favorite authors would do that, but tradpub doesn't play that way.
