The funny thing about most of the outrageously priced mid century modern furniture for sale in premium catalogs is that those pieces were designed to be affordable. A lot of those chairs were designed for public places like schools where you needed something to be durable and inexpensive because they might need to order hundreds of them.
There are some really good “replica” sites out there (they are all replicas by the way).
Another good source in the US is craigslist. Don’t search for Mid-Century Modern, or Eames though… search for “weird plastic chair” and you just might find a gem for next to nothing! The hunt is part of the fun. A friend of mine searched for “weird metal light” and she ended up getting an Arco light for about $200. An official reproduction would have been $2,500 or so.