1) What is the job market like for people who have TESOL/TEFL certificates? Will a certificate make a teacher eligible for any good paying jobs in the United States? Do you need a formal masters degree to teach in the States?

Requirements in the States vary according to whether you want to teach in a private or public school. If your intention is to teach in a public school you should check with the Dept of Education of the State where you are interested in working and ask them what their guidelines for employment are.

2) Are there many/any short term (3-6 month) assignments abroad? How long are average contracts?

A year contract is pretty standard but occasionally you do find schools that are happy to take on teachers for a shorter term. This usually happens when teachers leave unexpectedly or if a new course has just been introduced (business English, or company in-house) and they need extra staff.

3) Do many jobs you secure abroad pay well enough that you can bring money home?

It really depends where overseas. The Middle East for example is renowned for being a place where teachers actually make money! Check the Country Files in the ICALwiki www.icalweb.com/wiki to read about working condition, salaries and such like.

4) Does anyone have suggestions for a reputable, yet inexpensive, certification program?

Check what ICAL www.icalweb.com has to offer. You can read independent students reviews on TEFL Course Review http://www.teflcoursereview.com/ical-online-tefl-training. This a site where TESL/TEFL courses and course providers are independently reviewed and rated by those who have actually taken the course.