Something like this will work.
bill_amt = True
while bill_amt:
try:
bill_amt = float(input('First, what was the price of your meal?(Please do not use signs such as "$"):'))
except ValueError:
print('Please enter just a number')
continue
if bill_amt <= 0: #or if bill_amt has letters or symbols
print('Your meal wasn\'t $',bill_amt, '! Please try again.')
bill_amt = True
else:
x = float(bill_amt)
bill_amt = False