My friend had an old spanish coins date 1868 under reyna isabel, how much will it cost if he sell it?

Your best bet would be to take your coin to a local coin dealer. They will be able to tell you more by evaluating the condition of the coin. They will also know the amount minted and what it is going for in today's market.

You can also post pictures on this website (there are many coin experts here that can help you evaluate it - there also may be people willing to buy if you are trying to sell):

http://www.coincommunity.com/forum/default.asp

Speaking from experience, approximately nothing (that is to say of course, too little to make the trip to a coin dealer worth it).

Unfortunately, there is a widespread misconception that the value of the coin depends on rarity which, translates to being very old as in your friend's case. Nevertheless, rarity/age does not translate into demand. It is rather irritating that in such cases, there is a two-way market. If you want to buy, now the coin rarity comes to the fore and a dealer who may have it in his possession will certainly demand a high price for it. But if you try to sell the same coin to the same dealer, he will probably offer you very little for it. There may be valid reasons for it, such as that the dealer may have to keep it in his inventory for a very long time until a buyer is found. Most likely though, this is little comfort to people like your friend who may possess a true rarity which unfortunately no one is looking for.

Conversely, if your friend could discover let's say a one-cent U.S. coin in a very good state of preservation, with an unusual mint mark, or even a mint flaw during striking, etc., it could be worth thousands of dollars. The reason is that U.S. coins are in high demand by collectors and dealers in the U.S.

To sum it all up: Keep the coin. It may be worth little in money but a lot more as a memento of a trip, a special person you got it from, a lot of history, etc.

300 MM is a huge coin, you must have made a mistake. It would take more time than I allow to look things up to look up yours with just the small info you gave. I need to know exactly what is on both sides of it. A scan or photo is best but a good description sometimes is enough. I have too comment on what pelion said, I have never hear of so much junk from one person. Sounds like he has great command of the English language but knows little of numismatics.