case down-weighted to emulate equal presence and absences

Question

In GAM-based logistic regression pseudo-absence data is necessary to fit the model with corresponent presence data. In many cases, pseudo-absence data are generated 10 times more than presence records in order to ensure sufficient landscape variety in the pseudo-absences. To ensure that the presences and pseudo-absences contributed equally to the model and avoid bias, I would like to know how that case-weighting option can be get with R. For example, I want weight each pseudo-absence as n/1000, where n is the number of presence sites and 10000 are pseudo-absences sites generated randomly.