It is not obligatory to hold the Walima ceremony. Though it should also be kept in mind that Walima is the only ceremonial aspect of a wedding suggested by Islam.
Likewise, it is not necessary for the groom or his family to contribute in the Nikah. What exactly is this contribution for? If this is for festivity or entertainment then that itself is cultural artifacts not mandated by Islam. The spirit of an Islamic marriage is summarized as: the simpler the better.
The amount of Mahr is bride's right to decide and the groom does not have the right to set it unilaterally. It should neither be a value too high nor too low. There is difference of opinion among the different schools of Jurisprudence concerning the minimum allowed value of Mahr. You should consult a scholar for a detailed understanding of your situation.