The answer to this question - that is to say, to say what kind of source material is most appropriate for you to use - really depends on what your research question is and what you want to prove. I'm going to assume that you're looking for something you can use to help illustrate a point or inform your research for an undergraduate dissertation, in which case unless the focus is somehow specifically on pricing in and of itself, you probably don't need to worry too much about finding primary source material.

The key thing to keep in mind when doing work on slave pricing is that this is a complex and competitive market. You need to have a wide range of sources to get a grasp of the dynamics of pricing. The perceived value of a person held in slavery varied hugely according to a number of factors: age, sex, physical presentation, known skills, perceived disposition, intentions of the prospective owner in terms of the labour force, location and time of sale, and so on. This is in addition to the usual pressures that influence any commodity market: supply, demand, overheads and so on. For this reason if you want to make a point about price movements in the market for African slaves in the 18th century, you're going to need to pick your source material very carefully. For example, if you were working mostly with records in Barbados, you would probably find that the price average you reach is too low for the rest of the British Caribbean: it takes less time to get to Barbados than Jamaica. You must also be mindful that the historical record for the transatlantic slave trade is not nearly as rich as that of the 19th century southern United States and its internal slave trade; our understanding of the intricacies of the slave economy is much more fragmented in the context you're looking at.

If you just need a general idea about movements in the price of enslaved persons coming directly from West Africa, then the most accessible database that you're going to find is the Transatlantic Slave Trade Database; although it's only a small fraction of the total number of voyages, the scholars and researchers who put it together have found sale records for approximately 750 voyages in the 18th century. It is important to stress that these aren't raw figures, though. The price represents the average sale price of young, healthy men in contemporary pounds sterling. The price figures have also been adjusted to try and account for differential costs in transport: the price listed is an attempt to approximate the value of sale had the enslaved person been trafficked to Jamaica and sold there. This means that for someone who was taken to and sold in Barbados, the price has been revised upwards to account for the higher cost of transport to Jamaica. This adjustment isn't particularly distorting but does serve to try and make clearer to the reader where there is a real-term price difference in the sale value of two different groups of enslaved people, rather than a difference in the cost of transport from West Africa. You should keep this in mind when you look at the figures.

Beyond this, your best bet is simply to look for what other scholars have already written on this topic. I am reluctant to give you too much help in this regard given that one of the purposes of an undergraduate dissertation is to help you develop your own secondary research skills, not just your ability to find and analyse primary source material! Suffice to say that scouring journal databases for topics like 'slave price' should very quickly start turning up very valuable and interesting pieces of literature, and point you in the direction of further reading on the topic.

the price of slaves once they were transported to north america or britain

It's important to keep in mind that the slave trade for the British Empire was flowing into the Caribbean as an end destination, not into Britain itself. You are looking for what has been written about places like Jamaica and Barbados rather than Liverpool or London!