Chazal (Gemara Shabbat 118) promise, "Whoever makes the Shabbat pleasurable will be rewarded a portion without boundaries." The Bet Yosef (commentary to the Tur by Maran Rav Yosef Karo in Siman 242) cites the Mahari Abuhav (Rav Yitzchak Abuhav, Spain, 1433-1493) who explains that this reward is "mida k’neged mida," measure for measure. Just like one spent money without limit for the Shabbat, so too Hashem will give him a share without boundaries.
The Gemara also promises that he will be protected from subjugation to the government. This is also "mida k’neged mida." He accepted upon himself the responsibility and the yoke of the Shabbat, so Hashem removes from him the yoke and intrusion of the government.
Moreover, he is also rewarded that all of his heart’s desires are fulfilled. Why? Since he went out of his way for Oneg Shabbat, "mida k’neged mida", Hashem grants him all of his wishes.
Of course, one point has to be made. The Halacha states that even if a person prepares "kasa d’harsana", which is some type of cheap fish, like a sardine, that is also considered Oneg Shabbat. Why is such an insignificant dish, regarded as a misva?
The Shulchan Aish explains that Oneg Shabbat is determined relative to the means of the person. It’s a sliding scale. For a poor person, even a small fish is an important dish; it is considered as though he went beyond his borders. However, if a wealthy person would prepare such a dish, it would be regarded as a miserly lack of kavod to the Shabbat.
Based on this principle of "Relativity", the Shulhan Aish explains the term Oneg Shabbat-pleasure for the Shabbat, not for the person. The enjoyment is not for one’s stomach; it is for the kavod of Shabbat. The Shabbat doesn’t care if we eat sardines or filet mignon-as long as we have made an extra effort and investment in her honor. Therefore, everyone should stretch themselves for Shabbat according to their financial status. The reward for Oneg Shabbat is very great. The Aruch HaShulhan (classic Halachic work by Rabbi Yehiel Michel Epstein, 1829-1908) says that these rewards are not only in the next world, but in this world as well.
SUMMARY:
The misva of Oneg Shabbat is relative to the financial capabilities of each individual.
Whoever invests extra effort to honor the Shabbat, receives great reward in this world and the next.
