Twentieth century items can sell, but are so much more common that there is less demand. The eventual price will depend on a variety of factors and these are the same across many areas of collecting. So, rarity (knowledge of which will come with experience), condition, quality of workmanship, material, provenance and accurate dating are all important.
You could start a small collection of tongue depressors for a few pounds, but if you wanted to buy a top quality, signed, complete boxed 17th century surgical set with carved ivory handles you can expect pay in the tens of thousands.
