To be able to grasp the meaning of the line "Of labour you shall find the sum" in "Uphill" by Christina Rossetti, it is important to understand the poem's theme and structure. The poem consists of a conversation between two speakers. The first speaker is preparing to undertake an arduous journey on an uphill path and is asking questions about what might be found along the way. The second speaker offers reassuring answers, pointing out that although the journey will "take the whole long day," that the traveler will find a welcoming inn with comfortable beds and the company of fellow travelers.
The journey that the traveler undertakes is a metaphor for the Christian's journey through life. Although life sometimes seems to be a constant struggle, they will find rest and peace in heaven at the end of life.
In the first line of the last stanza, the traveler expresses concern about finding comfort because he or she will be "travel-sore and weak." In reply, the second speaker says, "Of labour you shall find the sum." This is a reference to the inn that this speaker mentioned earlier. In this line, the "labour" represents the effort that the traveler has made to get this far. The "sum" is the total amount of parts added together.
The second speaker is telling the first speaker that comfort will be found at the inn in proportion to the effort that the speaker has expended. In other words, the traveler will be able to fully and completely rest from all the exertions he or she has undergone. The comfort that the traveler finds will be equal to the effort.