It is my understanding that Heirloom varieties are non hybrid.
Perhaps way back some cherry tomato got frisky with a beefsteak but really who cares.
If you save a seed from a non hybrid seed and then plant it, you should have the same type of plant that you saved the seed from.
I own a book called...Seed to Seed by Susanne Ashworth.
Go HERE for a nifty blog post on seed saving. Understanding the mechanics behind the plants will really help you understand what hybrid and non-hybrid means. In the blog Sharon also mentions a book about plant breeding. It's a good thing to know too...say if you have a tomato you really like, but you've moved somewhere too chilly for it to grow right...you can breed it with a tomato that does well in the new climate and possibly have a great new breed that is just what you want.
There is nothing wrong with hybrids if you know what you are doing and are not depending on them for their seed to be true.
As for inexpensive seeds...it is my opinion (and I am full of them...) that you pay for what you get...
Seeds vary in prices for different varieties (rare ones are more money).
I would go with a seed company that has an excellent reputation for starters.
My favorite company is Territorial Seed Co and they have excellent quality and a reputation to match. Also if you are buying seeds for your future you don't necessarily need one of everything. This isn't like Noah's Ark and we aren't depending on just one person for the future of the world hehe.
Personally I would buy one packet per variety of seeds that you like and grow them and then save the seeds from them and grow them the next year and see what happens. Sometimes the best things come out of trial and error, but I digress