I don't think it's a matter of monetary greed but also having a greater 'power' than other believers, supposedly being more special than them. The Longer Ending to Mark is clearly a latter addition. Why would Jesus' disciples be tempting fate by handling snakes? It doesn't make sense. We're not to do miracles to bring glory to ourselves, but to bring glory to Christ.
The Shorter Ending of Mark
[[And all that had been commanded them they told briefly to those around Peter. And afterward Jesus himself sent out through them, from east to west, the sacred and imperishable proclamation of eternal salvation.]]
The Longer Ending of Mark
Jesus Appears to Mary Magdalene
[SUP]9 [/SUP][[Now after he rose early on the first day of the week, he appeared first to Mary Magdalene, from whom he had cast out seven demons. [SUP]10 [/SUP]She went out and told those who had been with him, while they were mourning and weeping. [SUP]11 [/SUP]But when they heard that he was alive and had been seen by her, they would not believe it.
Jesus Appears to Two Disciples
[SUP]12 [/SUP]After this he appeared in another form to two of them, as they were walking into the country. [SUP]13 [/SUP]And they went back and told the rest, but they did not believe them.
Jesus Commissions the Disciples
[SUP]14 [/SUP]Later he appeared to the eleven themselves as they were sitting at the table; and he upbraided them for their lack of faith and stubbornness, because they had not believed those who saw him after he had risen. [SUP]15 [/SUP]And he said to them, “Go into all the world and proclaim the good news to the whole creation. [SUP]16 [/SUP]The one who believes and is baptized will be saved; but the one who does not believe will be condemned. [SUP]17 [/SUP]And these signs will accompany those who believe: by using my name they will cast out demons; they will speak in new tongues; [SUP]18 [/SUP]they will pick up snakes in their hands, and if they drink any deadly thing, it will not hurt them; they will lay their hands on the sick, and they will recover.”
The Ascension of Jesus
[SUP]19 [/SUP]So then the Lord Jesus, after he had spoken to them, was taken up into heaven and sat down at the right hand of God. [SUP]20 [/SUP]And they went out and proclaimed the good news everywhere, while the Lord worked with them and confirmed the message by the signs that accompanied it.]]