Can I set a condition "IF a purchase request is over/under $ amount"?

Yes, open the Conditional Approval menu for the Approval Step concerned.

Only collect numbers on your Google Form. If the requester enters a currency symbol the conditional approval will not be met and the request will be sent to the default reviewer. 

For example, set a Response validation on your Google Form so that requesters get an error text when they enter $ sign

==>> How can I make some requests end at Step 1 and make others go to Step 2, Step 3, etc..?