I learnt that satellite can be equipped with transponder of different bands (L, C, Ku, Ka), and in term of bandwidth, L < C < Ku < Ka. My question is, am I correct to assume that, with a higher bandwidth, the equipment cost should be higher? i.e. Ka band transponder should be the most expensive and L band transponder is the cheapest. And a satellite equipped with only L band transponders should be cheaper to develop that that equipped with Ka band transponders. Thank you.
Hobbes is correct. Higher frequency transponders are more expensive to design and test than low frequency transponders. This is simply because as the wavelength decreases, smaller components within the transponder will have an effect (often unintended). You can build a VHF or UHF radio using a handful of cheap parts. The flip side of this is that higher frequency bands contain more bandwidth, so it is relatively easier to obtain the rights to a large chunk of Ka band than it would be in C or S band.
The transponder is often just a fraction of the entire mission, so it's cost is usually insignificant or minor in comparison to other costs. Also, since higher bandwidth satellites can deliver more data to more users, the cost of the transponder is quickly repaid. Think about the number of users dish satellite has and the revenue they generate.