Make enquiries from local dealers and get average price.
Search online from local dealers to get average price.
Search online from dealership oversea e.g. cars.com, autotrader.com to get average price, then add dealer fees, shipping cost and estimated clearing cost in Nigeria to arrive at final average. This is just to serve as a guide in knowing the cost from source, as most dealers who ship cars to Nigeria get them through Auctions, which are way cheaper than buying from dealership.