Оксана, вот так примерно:

Dear XXX,
I have very much enjoyed our communication for the last XXX days/weeks/months. I appreciate your invitation to visit you in your country.
Unfortunately, I cannot afford the trip to your country due to my income is limited.
However, I am interested in getting to know you better. I encourage you to visit my country for a few days so we can figure out whether we have a chance to move forward with our relationship.