The gemara learns out from the fact that the mitzva of hashavas aveida would require someone to save someone's life, that the explicit pasuk of לא תעמוד על דם רעך would demand even the spending of money (which would not be required by the mitzvah of hashavas aveida). The Rosh points out that if I indeed spend money to save someone's life, he is obligated to reimburse me, but even if he can't I am obligated to spend the money in order to save him. The Rosh learns this out from the gemara on 74a which says that if the "nirdaf" (one being chased to be killed), breaks the vessels of the "rodef" (chaser) in the process of escaping, he does not have to reimburse the loss of property, but if the nirdaf breaks the vessels of someone else, he must reimburse him. This shows that a bystander isn't obligated to pay for the saving of the nirdaf at his own expense when the nirdaf has money, because if the bystander would be obligated to pay to save the nirdaf, then he doesn't deserve to be reimbursed for the vessels that the nirdaf used to save himself.
How does the gemara know that the pasuk of לא תעמוד על דם רעך demands the spending of money (even if the person wouldn't have the financial ability to reimburse)? Rashi writes that the pasuk of לא תעמוד על דם רעך says, do not stand on yourself and hold yourself back from saving someone, rather - חזור על כל צדדין שלא יאבד דם רעך. The pasuk demands to do everything that is within your power to save someone's life, even if it will cost a lot. Rav Moshe (Igros Moshe Y.D. 1:223) points out that the normal limit of how much one has to spend on a mitzvah doesn't apply here. Rashi holds that the pasuk is specifically addressing how much one has to spend and is demanding that one spend whatever necessary to save someone else's life.
From the fact that Rashi had to darshen the pasuk and interpret it to be specifically addressing the expense, Rav Moshe has an amazing insight. There is a lot of discussion regarding the limitation of 20% that we are required to spend on positive mitzvos. The Rama in Hilchos Sukkah seems to hold that it only applies to positive mitzvos, but regarding negative mitzvos there is no limit to how much one must spend to avoid doing an issur. Rav Moshe proves from Rashi that the primary distinction is not whether it is a positive mitzvah or negative one, rather whether it is violated actively or passively. If the halacha was that for any negative mitzva, even if violated passively, one would have to spend everything then there would be no need for rashi to darshen the pasuk of לא תעמוד על דם רעך to demand spending money. The very fact that he would be in violation of a negative commandment would require the spending of money. From the fact that Rashi finds it necessary to darshen the pasuk to be saying explicitly that money must be spent, the implication is that for this type of lo ta'aseh he wouldn't have to spend more than 20%. Why? It must be that Rashi holds that a lo ta'aseh that is violated passively, similar to most positive commandments, only demands a maximum spending of 20%. It is only because Rashi darshens the pasuk to be explicitly demanding חזור על כל הצדדין שלא יאבד דם רעך, that he would be required to spend more than the 20% maximum.