It's true that you can take a TEFL course over one weekend, but the 'qualification' is not worth the paper it's written on. You certainly won't learn to teach in one weekend! To be honest, if you wanted to learn a language, would you choose a teacher who had 'qualified' over the weekend? Thought not!
There are two types of TEFL courses - online courses and face-to-face courses. Online courses are self-study rather than taught courses. You do the work at a time that suits you and submit exercises to an online tutor who will mark them and provide you with feedback. Some online courses include a 20hr classroom element (the weekend course that you mentioned above), and though many trainees find it useful, it cannot be equated to the observed teaching practice mentioned below. Prices for online courses vary, but you should expect to pay between £200 and £500 depending on the length of the course.
Some schools will only accept face-to-face courses, such as the CELTA or Trinity CertTESOL. These are the 'gold standard' of TEFL courses and are recognised and accepted worldwide. They are taught courses generally done intensively over four weeks (although it is possible to do the CELTA part-time over six months) and they include the all-important 6hrs of observed teaching practice. At around £1,000, they don't come cheap. They will however open more doors than an online TEFL certificate ever will. If you want to teach in Western Europe they are recommended, and if you want to teach in a native English-speaking country, they are essential.
That said, to teach in China an online TEFL certificate will more than suffice. The best advice I can give you is to do the longest course you can afford. Bear in mind though that the industry standard is 100-120hrs, and that you will be competing for jobs against people who have this. Demand for EFL teachers in China is high, and currently you do not need a degree or experience to teach there.
Hope that helps.