Cracking coin, you lucky devil! I've always fancied trying my hand at metal detecting, but lack of field sites and other distractions have prevented this. Offa coins are not numerous and much sought after by collectors. Out of my legue though. I don't have an up-to-date catalogue, but a trip to the local library to look at the latest Spink Coins of England would give you an idea. My2004 edition gives the value at £750 in fine, but things have moved on. I'd put your coin at Fine, mainly because of the pitting but this is subjective and I'm sure there are masses of people out there who will drool at the sight! Congratulations on your find!