Check this website about the dangers of speculative work. Very helpful.
from the NO!SPEC website:
Why is spec work unethical?
The designer in essence works free of charge and with an often falsely advertised, overinflated promise for future employment; or is given other insufficient forms of compensation. Usually these glorified prizes or “carrots” appear tantalizing for creative communicators just starting out, ending with encouraging examples like “good for your portfolio” or “gain recognition.” The reality is that they often yield little extra work, profit or referrals. Moreover they often must sign a contract unwittingly waiving their valuable creative rights and ownership of their work to the ones promoting this system. A verbal agreement is ineffective in protecting the rights of the designer in a court of law. As a result the client/employer will often employ other designers using similar unprincipled tactics to change and/or resell the creative work as their own. This also promotes the practice of designers ridiculously undercharging themselves in the hopes of “outbidding” any potential rivals, devaluing both their skills and those of the graphics industry in the process. Promoting this method encourages some clients/employers to continue preying on uninformed creatives for menially valued labor.