This paper reviews a couple of simple and low-cost techniques suitable to interrogate the fiber Bragg grating sensors used in structural health monitoring and provides few examples on their applications. Both these techniques evaluate mechanical strains through the measurement of optical power variations, thus avoiding expensive spectral analyses. With the first technique, which uses a broadband source and a pair of matched gratings, it is possible to fabricate a low-cost optical sensor with intrinsic compensation for temperature effects. The other technique, which is based on a laser source wavelength-matched with the Bragg grating, allows evaluating dynamic strains down to few nanostrain. These two complementary approaches are analyzed and compared, highlighting their strengths and weakness from the point of view of structural health monitoring applications. Then, since interrogation methods that exploit the variation of the received signal amplitude are very sensitive to noise, a description of how performances can be improved using standard signal-processing algorithms is also given. The combination of one of the proposed approaches with the suitable signal-processing techniques allows achieving results comparable with other common grating interrogation methods, but at a fraction of their costs.