Speaking specifically of Downton Abbey, they rent out farms on their land which are producing crops, animals, etc. They charge rent to these farms which is either paid in money or goods (or both). They also own most of the little village shown on the show which forms part of their estate holdings and they charge rent to all their tenants. This is a classic "landed gentry" situation. Meaning that they own large parcels of land gifted to their ancestors by some king or another for good deeds, military service, being related to him, whatever pleased him. They rent out the land to tenants who pay taxes and provide their goods and services in able for the estate to survive. I can't speak to a normal income, but the idea of being "landed gentry" means that the profits from your estate sustain the estate, house, and lifestyle so that no one in the family has to lower themselves and actually be employed (i.e.: politics would be an exception if they so desired) so clearly the estate has to make enough money to pay for the lavish lifestyle, servants, travel, clothing, food, etc.

However, not all estates thrived. Through mismanagement, poor land, overspending, estates could fall into major debt resulting in less money for the family. In general, the bigger, more arable, better managed, well located, the estate was, the more the estate could make, the wealthier the family was, the better lifestyle they could live, the better position in society they could hold.