I'm a tax preparer with a small number of returns. What software is the most inexpensive and best for me to use?

Paid tax preparers cannot use TurboTax.  It violates the terms of service.   If you are being paid you need to use professional tax software.

https://proconnect.intuit.com/?s_cid=TT.com_MoreProds-Footer_ProTaxSoftware

If you mean you have several returns to prepare for your own family members, etc.----that's different.   You should use desktop software so that you can e-file up to 5 federal returns.   You would also get the software for one specific state with the ability to purchase and download other states if needed.

CD/DOWNLOAD (This requires a full PC or Mac—-cannot be used on a mobile device)

https://turbotax.intuit.com/personal-taxes/cd-download/

You can also purchase the CD/download from various retailers such as Costco, Amazon, Walmart, Best Buy, Staples, Sam’s Club, etc. etc. etc.

This link Where can I obtain authentic TurboTax software? has a list of authorized TurboTax resellers.

When you use desktop software, there is an additional fee to e-file a state return.  You can avoid that fee if you print, sign and mail the state return.  The state e-file fee is $19.99 and increases in March.