There's been considerable discussion on the subject in past threads on the forum.
The "1 credit = $50" equation comes from the RPG handbook, which, though licensed and recognized by Frontier, is not necessarily canon lore.
Attempting to work out a credits-to-dollars conversion rate by comparing prices of commodities is tricky, because the comparisons depend entirely on which commodities you use. If you use gold, then a credit is ridiculously cheap - that's because, in the ED universe, gold itself is ridiculously cheap, because it's ridiculously common. For example, it's estimated that the entire human race, over the entire course of history up to the 21st century, has mined and refined about 200,000 tonnes of gold. A single player who likes going mining could mine that much in just a few weeks, just by going to a mineral-rich system and shooting lasers at rocks. Or, to look at it another way, right now in the Duamta system, the four mining stations have between them about 200,000 tonnes of gold for sale - the entire 21st-century gold stockpile of Earth, and you can buy it and take it away if you had a big enough ship.
The things we in the 21st century call "food", on the other hand - real meat, real fruit and vegetables, real fish, real beer - are expensive, labour-intensive luxuries in the ED universe. Most people, most of the time, eat cheap-but-nutritious 3D-printed pseudo-food generated by autochefs using Food Cartridges.
But the general consensus from past discussions is that we spacefarers are the upper crust of society, incredibly wealthy just by virtue of owning a spaceship. The average Joe, one of only 6.6 trillion people in the galaxy, might dream of travelling into space on a holiday one day or even emigrating to another planet, but actually owning a spaceship will be beyond them; for the most part, they're stuck on whatever planet they were born on.
The "average salary" in the ED universe is lower than it is today. This is manifested in the form of so-called "wage slavery" in the Federation and other liberal democracies, and actual slavery in the Empire and Independent spheres. In a sense that's OK, as many of the things which we need money for today - food, clothing, personal consumables, etc - are cheaper and more readily available in the ED universe, thanks to galaxy-wide economy of scale and technological advances (such as the autochefs mentioned earlier). Very few people in the ED universe are actually poor, by 34th century standards. But it does make it virtually impossible for people to "save up enough money" to buy a starship.