Depending on what exemptions you get, you’ll be starting with the Fundamental Level papers. With a B.Sc in Economics from a Nigerian university, you may be entitled to exemptions from papers F1 to F3. You can find out how many papers you’ll be exempted from here. Remember though, even if you take exemptions, you will be expected to pay for the papers you are exempted from, so it is not a saving.
Before you take exemptions, I would suggest you brush-up on the knowledge from those papers, as each subsequent paper builds up on the knowledge from previous papers. Read this article on exemptions.
If you are a Nigerian student, you don’t need to pay your fees in USD or GBP. ACCA Nigeria allows you to pay all your fees in NGN – contact Nigeria.firstname.lastname@example.org for bank account and payment instructions. To find out expected cost of registration, exemptions and other fees, check here.
On lectures, there are free resources available online. I’m also asking students taking F1-F5 to send in syllabus topics they would like to see covered, like I’ve done here & here. I passed all (except 2 papers) of my ACCA exams by studying on my own, so I know it is possible.