The sages say that the more economical option is only effective if a person is genuinely poor. A person who can well afford the regular offering will achieve nothing by trying to get away with giving a poor man’s offering.
The Chafetz Chayyim broadens the discussion by saying that this principle applies to the performance of all the commandments, not just the Biblically-ordained offerings. There are often emergency provisions for people who are not in a position to carry out the full mitzvah.
When time is pressing, for example, there is a short form of Amidah that can be used. But someone who is not really short of time but is just looking for a bargain, for a minimal way of fulfilling the commandment, is abdicating his responsibility and will not get away with it in the eyes of God.