Hello.
First of all, you have to understand that creating a compiler to modern C++ is very hard and challenging task that requires years of work. I do not want nor I think I can do that myself. So I'm talking about a C compiler.
Even for C, modern C is hard to parse and emit code for and emit efficient code is even harder. If you want an industry-level compiler, with good error messages, support for C11 and good optimizations is not something I can do with the above time and budget. I can do this, but double the price and time to the zone of above €3,000 in above a year. These are all estimates - if that what you want (and consider seriously whether this is really what you want: no matter the reason, using an existing compiler will always be cheaper than developing from the ground up. If you find this suitable to you, we can discuss about the costs privately).
The above estimates (€500 in a month) assumes C89 compliant compiler to x86 (or x64, ARM or ARM64) without optimizations. Standard library included, but not all of it (common parts only - again, chat privately).
Again, think seriously about your needs. Odds are very good that you can use GCC or Clang. If this is a CS task, probably you took the wrong language. Developing an interpreter for BASIC would be much easier.
Best regards.