I used 10 prices for each element, once 10 was filled, I continued to replace always the highest price with new lower market prices since market prices change constantly so really old high values would skew the average higher and as seen in stalls, elements with high prices don't sell. There still few that don't have even 10 prices listed just because I haven't seen them on sale in stalls. Even if those averages aren't really accurate statistically, they've helped me a lot to see on what prices to buy and for what to sell (since I can check my price lists, I can see also minimum market prices

). I could change my data to hold 30 or even 50 prices for each element to get more accurate results, but I'm not using that much my time to collect that data amount
I also have charts how much elements cost for each tablet so that I can see how much I'm "spending" on alchemy
And remember, those are only from Babel server, price ranges might be totally different on other servers.