If ばかり could mean approximately; about; and could also mean only; merely; nothing but;, then how should we know if this sentence ５０００円ばかりもっている。 means:
1) I have about 5000 yen.
or 2) I have only 5000 yen.
Note that ばかりに, ばかりか etc. also exist and work differently.
As @Scott just pointed out, in your example, you would use だけ or しか (+negative) to mean "only".
If the verb is past tense it means "just" happened. If the verb is -te iru it means only.
There are about 3 or 4 other uses that arent used as much. bakarini nado...