Midrash su Levitico 27:8

4) "then he shall take one he-lamb as a guilt-offering": R. Akiva asked R. Nechemiah: What is the intent of "one"? He answered: This one (the leper) brings according to his means and one who atones for sanctuary uncleanliness brings according to his means. (I might think that) just as the latter brings two birds (a sin-offering and a burnt-offering) instead of one (ewe-lamb) as a sin-offering, so, this one (the leper) brings two (birds) instead of the (ewe-lamb) sin-offering, (one for a sin-offering; the other for a burnt-offering), and the beast burnt-offering, (for which the rich man brings a he-lamb for a burnt-offering) is binding (also upon the poor man to bring, so that he will have to bring two he-lambs, one for a guilt-offering and one for a burnt-offering.) It, therefore, must be written "one he-lamb as a guilt-offering," (and not more.) R. Akiva said to him: From "the place that you come" (that he must bring two, I can deduce that he brings only one, viz.) This one (the leper) brings from his means, and the sanctuary defiler brings from his means. Just as the latter brings two for (the sin-offering, which is) his atonement (and he need bring nothing more), so this one (the leper), need bring only (these) two (birds) for his atonement, (and nothing more)! He answered: If not (as I say), how do you satisfy it ("one he-lamb as a guilt-offering")? He answered: This one (the leper) brings from his means and in valuations one also brings from his means (viz. [Vayikra 27:8]: "According to the means of the vower shall the Cohein valuate him.") (I might think that) just as there he brings whatever he can attain, so this one brings whatever he can attain, (so that if he can attain two birds and two sheep he should bring them); it is, therefore, written "one" (he-lamb. Even if he can attain two sheep, he brings only one he-lamb as a guilt-offering, and two birds, one as a burnt-offering; the other, as a sin-offering).

6) I might think that I (also) exclude (from valuation even) an organ that one's life is dependent upon; it is, therefore, written "souls" (i.e., if he dedicates such an organ, he pays the value of his entire "soul.") "souls": and not (if he dedicates the value of) a dead body. I would then exclude a dead body, but I would not exclude one who is at the point of death. It is, therefore, written (Vayikra 27:8) "then he shall be stood … and the Cohein shall valuate him." One who can be "stood" is subject to valuation; one who cannot be "stood" is not subject to valuation.

13) (Vayikra 27:8) "And if he be too poor from your valuation": If he is too poor to assign a valuation, "then he shall be stood before the Cohein." What is the intent of this? Because it is written "souls," to exclude the dead, I would exclude the dead, but not the dying; it is, therefore, written "then he shall be stood … and the Cohein shall valuate him" — Where there is "standing," there is valuation; where there is no "standing," there is no valuation.