What does this line mean from Milton's Paradise Lost?

Question

so should I purchase dear / Short intermission bought with double smart. (Paradise Lost, IV, 101-102)

Answer 1

Satan is upset that he is in Hell, and questions whether he should apologize about his rebellion and be allowed back into Heaven. He realizes that this would only last so long before he would feel the need to rebel again, and that he would just be cast out of heaven and fall again, and this time even harder. The "dear short intermission" he would get by returning to heaven would require the price of him falling a second time, or "double smart".

Answer 2

Here is the complete thought:

But say I could repent and could obtaine
By Act of Grace my former state; how soon
Would highth recal high thoughts, how soon unsay
What feign'd submission swore: ease would recant
Vows made in pain, as violent and void.
For never can true reconcilement grow
Where wounds of deadly hate have peirc'd so deep:
Which would but lead me to a worse relapse
And heavier fall: so should I purchase deare
Short intermission bought with double smart.

This means:

Let's say I could repent and return to my former place in heaven. I'd quickly have the ambitious high thoughts I had before -- in part because my repentance made because of my pain would be forgotten once the pain is gone -- and I'd fall again worse than before. Therefore it would cost me much ("purchase dear") to get only a brief relief from hell ("short intermission") and the cost would be twice the pain of before ("double smart" (smart as in, that smarts)).