I have no evidence that blasting no-follow comments with scrapebox on thausands of blogs is going to help your rankings, even for low-competitive kws. If you think that clicking the "post comments" button on scrapebox is going to give you automatically good rankings, you are delusional.
Everybody have scrapebox nowadays and a lot blast comments like there is no tomorrow and their comments sucks big time; blogs have been blasted for some time now and blog owners aren't that silly, they won't approve your comments or they will, but with no link.
Plus, you will need private proxies to be able to post effectively, cause public proxies are slow and probably have already been reported for spamming, i.e. useless. So, be ready to shell out 10-20-30 dollars every month for private proxies, depending on the vendor you choose.
said so, Scrapebox is a very good tool since you can scrape google, you can check PR, check links etc, it's a tool that can come in handy often while doing your thing online.
but if you hardly have 60 bucks to invest and you hope scrapebox is going to pave the way for the thausands, i think you are going to have hard times making it. Nothing is impossible, you know but...
if you want to rank for a low-competitive kw, in terms of cost-effectiveness you may want to build manually 50/100 profile links a day and I can ensure you'll see much better results that blasting the shit of the wordpress blogosphere.
first establish some cashflow, then once you are making some money on a daily basis and you have some cash saved to leverage your online activity, go for scrapebox.
don't be a noob with scrapebox.