Ah, I misunderstood the question. I thought he thought that the solomonoff prior wouldn't be normalized - so for example, a program of length 30 and a program of length 33 would both be in infinite strings, so as you search infinity strings you find them equally common.
Still I don't understand the "exponential" part. I thought that you may have deliberately given an obscure brief answer to the obscure brief question in the OP.
Why wouldn't the probability of two algorithms of different lengths appearing approach the same value as longer strings of bits are searched?