To arrive at this amount, I use the calculations that I borrowed from Joe_Nobody of Kindleboards. I already discussed this before I think, but in case you don't want to search for it in my archives then here it is again.
(COMPUTATION BELOW IS FOR SAMPLE PURPOSES ONLY)
Monthly Income Target: P90,000 (this should leave you with about P50k every month once taxes and advertising expenses are deducted)
Estimate royalty per $2.99 (my regular price for ebooks): P86
Average number of copies sold per book per day: 5 across all retailers (let's aim for a really small number, meaning even if you don't lift a finger, you will still surely sell this number per book each day)
Royalty per book per day (P86 x 5): P430
Daily Income Target (P90,000 / 30 days): P3,000
# of books you need to publish in your lifetime (P3000 / P430): 6.97 or round up to 7 books.
To be safe, let's just say you need 8 books in total. Eight books priced at $2.99, selling FIVE COPIES A DAY EACH. If you can manage that, you can then earn a gross monthly income of P90,000 per month.
Right now, I am at 19.8% of my target number of books to be published in this lifetime. Knowing this number makes me excited and it definitely gives me motivation to write. One reason why I want to reach my target quickly because when I do, I will have more time not just to spend with my family and loved ones but it also means I'll have more time to write in advance. I can write an entire series and then release it all at once, which I know is how readers usually prefer to read books in series (I'm kinda the same as well hehe).
So, yes, I was a little depressed earlier, but now I'm not. :D Off to writing so I can be another book closer to my target. Wee!!!