My Solution

@prices = []
@num_of_items = 0
@sum = 0

puts "Enter the amount of money you have:"
print "$"
@amount_of_money = gets.chomp.to_i

puts "Enter the number of items:"
@num_of_items = gets.chomp.to_i

while @prices.length <= (@num_of_items - 1)
    puts "Enter item #{@prices.length+1} price:"
    print "$"
    @prices << gets.chomp
    break   if @prices.last == -1
end
puts "*" * 50

@prices.each do |p|
    @sum += p.to_i
end
puts "You have a total of $#{@sum}."
if @sum >= @amount_of_money
    short = (@amount_of_money-@sum).abs
    puts "You have enough money by $ #{short}. Now go buy it!"
    exit
end

if @sum < @amount_of_money
    diff = (@sum-@amount_of_money).abs
    puts "Sorry, you don't have enough money. You are short by $#{diff}."
    exit
end