The reason may be that Epson has taken steps in recent times to curtail the sale of compatible Epson inkjet cartridges.
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About £4, for instance, for a full set of six compatible cartridges for an Epson R200 or R300. These are Sumvision branded compatibles (the firm noted for MP3 players and similar products).
The same cartridges appear to be rebranded by other companies but at higher cost.