Your 1775 coin is not a Penny, one was not minted that year. I suspect you have a 1775 Halfpenny or less likley a Farthing. If what you have is a Halfpenny (about 30mm in diameter) you are going to need to figure out the grade to determine the aprox. value. Other factors that must be taken into consideration are toning, luster and eye appeal. Two coins that are both Unc. might have widely different values based on the most subtle of differences. For example if your coin is a fully red Unc. it is going to command much more than a brown with ugly color. Some red-brown and brown coins take on a blue color which many collectors seek out and are willing to pay more (often much more) for. If the coin you have has seen circulation, the prime factor to consider is the amount of wear that it has. If most of the design is still there and only the high points are rubbed away, it will be worth more than a coin that is worn out. As your question stands, your 1775 coin (if it is a Halfpenny) could be worth anywhere from 50p to £400. Condition is everything! PLEASE DO NOT CLEAN YOUR COIN, Doing so will ruin its value to a collector.