It's not about 'expensive' as such. I work on a 'value for money' principal and FreeAgent saves me valuable time that I used to spend mucking about with spreadsheets and trying to stay on top of invoices and especially recurring payments (such as hosting and domains). I do about 15 minutes on FreeAgent every day and then I am free to earn money. It is so intuitive to use that it ticks all the right boxes for our business. Plus you can take away the cost of needing an accountant and FreeAgent more than pays for itself.
I'll take a better look at Salestastic when I get a chance but it would take a lot to move us away from using or recommending FreeAgent right now.