You will have to do serial dilutions for the compounds and prepare a cell viability assay. So, you will need some sterile microcentrifuge tubes for dilution purposes, a few cell plates with a high number of wells to put your cell lines in & inoculate your compounds, as well as selecting an assay you want to use to determine your IC50. You can select one out of a few assays you want to use here: https://www.ncbi.nlm.nih.gov/books/NBK144065/

The costs for these is highly dependent on your distributor, state & country, especially since prices for plates, tubes and assay kits differ significantly even among distributors. If you do not have a cell line, its media, a micropipette for dilution and inoculation and said compounds ready at your institution, you will have to buy those too. Worst case scenario is if you also do not have a spectrophotometer to read your assays. So, I would suggest finding out your local distributors that do sell these basic medical laboratory materials, and just email them for an invoice of what you don't have. I won't be able to tell how prices there are like, but do be prepared to allocate a large amount on your spectrophotometer, after that would be your micropipette, then cell line: one spectrophotometer can cost thousands of USD, one micropipette can cost hundreds, one frozen cell line vial will definitely hit more than 50USD.

Getting in touch with the biology/medical department of another institution or yours would be much easier, they would know the estimate prices, and likely have the equipments as well. Definitely do not recommend buying an entire spectrophotometer for an one-off experiment.

Assuming that you already have access to wet-lab basic equipment (pipettus, micropipettes, centrifuges, biological hoods ecc.) and that you already possess the cell line of interest, some plastic (pipettes, flasks, plates...) and growing medium, and most of all a spectrophotometer, you will need to buy all the compounds (at least 250€x 20= 5000€). This may be the raw price (very generic) for only the chemicals, and if you need to buy the other consumable supplies mentioned above you could add 2000€, add more 1500/2000€ per month for every employee that will do the work and 15-20K € for a micro-plate reader if you have not. As you can see it depends on several things, so, you should try to make a list of what you have access to and compare it with what you need, before doing the math. You should also speak with your lab manager to get a better definition of the costs.

Can you find a collaborator who has published using the cell lines you want? Maybe seek collaboration for a co-authored manuscript? Please make sure that you test a control compound along with your compounds. You control should ideally inhibit the same pathway as your compounds - or if the mechanism of action of your compounds is unknown, choose a standard chemotherapy drug that inhibits the cell lines you're testing against. Another factor that can alter IC50 in in-vitro tests is the amount of competing natural substrate used in the test - for example, amount of ATP in kinase inhibitory assays.