ViaBTC accelerator services will require payment for their services, so it's not free. Yeah, it's not worthwhile unless your amount is small or if you're tight on cash. For example, if I have a substantial withdrawal, let's say $4k, they might demand almost $500 or even more, which is very high. However, when the amount is low, perhaps below $100, the fee becomes lower.
ViaBTC does offer 100 TXs to be accelerated for free of cost if the size of the TX is below 500bytes and the fee is over 10kb/byte but you can't find a spot in the 100 per hour when the network is congested.
And the fee for acceleration is based on the size of the transaction not based on the value as far as I know and its not really a portion like you said.
For example, look at this below,