Hi naazira
Power budget of a system is the difference between the transmitter power and the receiver sensitivity of your system.
And you can check the reliability of your system by comparing the value of your power budget with the total losses in your system.
Make sure that your power budget is high enough to cater all the losses.
We cannot generalize the value of the budget to be high or low.Some systems demand a high value of budget which means that they are using high power transmitters while in some systems it is not possible to use high power levels and power budget of these systems will be low.
Regards