a decent living is possible but risky - you would need to be self employed and usually rent space (or the ability to training clients) in health centres or gyms... remember there are thousand of personal trainers out there so compition is high and as the industry becomes more regulated you will need qualifications and proof of proficiency to operate within the larger chains. My educated guess is that 30 thousand would be achieveable but very very hard sum to produce, remember that you will have to work the most unsociable hours (e.g. evening, very early mornings, weekends ect.) and will have to be physically very fit - which can be hard to maintain. Add to this the risk of any injury, which could put you out of work and you risk a varying income.