What are the differences between meritorious acts and wholesome acts? A detailed answer with references is highly appreciated!
When we discuss virtuous living and behaviour according to the Buddha's teaching, two special Pali words are used - kusala and punna. Kusala has come to mean 'karmically wholesome', while punna is taken to mean 'merit'. 

Often Buddhists in Sri Lanka use the two words to describe one and the same kind of activity - "meritorious". However it would be good to look at these two words in the context of the Buddha's discourses. 

There is a difference in the two words, and have to be taken in the context in which they are used. The Buddha has said, "Accumulate much wholesome actions." He has used the term 'wholesome' in the practice of the Noble eight-fold path, and development of tranquillity and insight. 

Merit is the term often used to describe actions that bring about spiritual welfare and contentment. He has discoursed, "Bhikkhus, 'pleasant disposition' is another word for 'merit." That would mean one should engage in meritorious deeds to receive a life of comfort and contentment. He has also said that it is an auspicious event in life to be possessing previously accumulated merits. 

There is a view among a few lay people as well as monks today that since merits prolong the process of existence and the round of rebirths, one needs to accumulate only wholesome actions and not merit. One gets into debate about merit and wholesome actions due to non understanding of the reasons for the continuance of the round of rebirths. Round of rebirths occur due to Dependent Origination. 

If the continuance of the round of rebirths occurs due to Dependent Origination, the latter must cease for the round of rebirths to cease occurring. The way to make that happen is to follow the Noble Eight-fold Path. To fulfil that process, one needs to association of noble friends, hearing the Doctrine and keep company with righteous people. He would also need a rebirth conducive to fulfil the process of following the Noble Eightfold Path. Now, that is where merit plays its part. 

A rebirth in worlds of the Brahmas and Devas becomes a very helpful asset to one who searches for the goal of Nibbana. Such rebirths are the results of merits. So we must understand the true worth of merit. Those who spend the time arguing "We do not need merit but only wholesome deeds" unfortunately may end up getting neither. 

Undoubtedly, it would be a wholesome action to have a thorough understanding of Merit and wholesome actions before one would discuss them. Trying to discredit merit can only result in accumulating unwholesome actions. The Buddha has this to say about merit: "If one has enthusiasm to do a meritorious deed, do so over and over again. Develop a desire to do merit. Merit is comfort." 
The Buddha's consistently emphatic message is to annihilate desire. However, He exhorts desire for merit because merit is needed to make our lives comfortable and conducive to search the goal of Nibbana. Our birth in this world and all our good achievements are results of accumulated merits. 

There are those who may say that in today's world many things need money and therefore even doing merit can be expensive. However that may be, money and merit have no real proportion. Merit is something that does not measure and count with money. Merit is based on a non-aggressive and virtuous mentality. 
Merit has its origin in right thinking. It is the right thinking that teaches us the benefit of selfless giving, benefit of merit, that good and bad actions have similar outcomes, there is benefit in taking care of one's parents. To say that one cannot do meritorious actions for want of money would not amount to right thinking. 

Observing the five precepts, observing the eight precepts on Poya day, Contemplation of the Buddha, Contemplation of the Dhamma and Contemplation of the Sangha, and meditation on loving kindness do not cost money. In short, we can earn merit even from the dish water we throw away. Thus the talk that merit is costly may be misleading to those who don't have money as well as to those whose wealth is more than they can count.