How do the completion of online courses impact a Master's application?

So if someone mentions those kind of course certificates in CV or application material for grad school, does it have a significant value for the applicant?

They are better than nothing, but it's unlikely that they would have significant value. Courses are only a small part of what matters for graduate admissions, and elementary or low-level courses matter the least of all. Only a fraction of Coursera/Udacity courses are at a high enough level to matter, and even those courses aren't likely to make much of a difference.

I would not recommend devoting any time to online courses for the purpose of graduate admissions. If they are teaching something you really want to understand and have no better opportunity to learn, then that's a good reason, but the learning will have to be its own reward.

Do you consider these kind of certificates at the same level with a grade on an academic transcript?

I'd look at a certificate of completion for an online course the same way I'd look at a traditional course taken pass/fail (i.e., without a grade). It's evidence that you have done something, which shows some level of motivation and energy, but it's not evidence that you actually learned much in the process.

However, there's a bigger issue than grades here. Ultimately, good grades don't mean that much: standards vary dramatically, the ceiling is rarely high enough to distinguish between excellent students, and even if the ceiling is high enough it's not clear that this is a meaningful comparison. Letters of recommendation are crucial for supplying the information grades alone can't supply, and this is something MOOCs are currently unable to help with.