Affordability often comes at a price- many of the cheaper courses are pretty worthless as qualifications and are not widely recognised, or a tied in to a particular school, which restricts the transferability of the qualification. I would also look very carefully into the recognition and validation of the courses- there are courses that self-validate by setting up an essentially bogus organisation or using one where the basic criterion for validation is the ability to put a checque in the post.
If you find one yo're interested in, then post details here and I'll happily take a look. I would recommend the CELTA as a starting course simply for its acceptance, which is worldwide, though I realise that this one is not in the easily affordable category.
Student or Learner