Bass strings in modern pianos are copper-wound strings (see picture). Each one are in different length, warped by various number of layers and thickness of copper wire.
If a bass string of your piano is broken, there are 3 main ways to fix it: ordering new replacement strings, splicing the broken string or simple removal.
Ordering New Replacement Strings (Most Proper)
This is the proper way, especially for newer and expensive pianos.
It is ideal to order the replacement string from the piano factories, however dealers in Hong Kong don't offer such service (I have asked). I have only been able to order new string directly from Sauter in Germany, where I previously had practicum.
Since the string is brand new, it won't break easily. Therefore, this is the best long term solution. A new string also sounds better.
String duplication requires the broken old string, and will take some time.
A string will stretch over time, therefore a new string will go out of tune sooner, requiring multiple tuning before it will stay in tune.
New string's outlook will shinier than old ones, more obvious on grand pianos.
Due to various difference (i.e. material), the sound of a new string might differ from the old string.
The fee is higher due to requiring both new material and labor. It also takes longer time to complete.
A new string costs about $200. If a technician needs to come take it off the piano, it will count as a service. Therefore, when possible, customer may bypass this service by removing the broken string themselves, then bring or mail in the string to a specific location for string duplication.
That says, the minimum total cost for replacement one bass string is $600 or up.
Splicing Broken Strings (Quickest)
This method is a technical task that can immediately revive a broken string, which might last a long time.
The success rate is about 50%, due to the fact that the already-broken string might break again somewhere else.
Even a success splice might fail. The spliced string might have metallic noise.
It takes 30 to 45 minutes. Work time is counted whether the splice is successful or not.
Simple Removal (Cheapest)
Some bass note consists of two bass strings. When one is broken, another will still make sound.
This is the cheapest way to restore piano's function.
A single string instead of two will have less volume and different tone color. Sound dampening of that note might be affected too.
This method doesn't work for single-string note.
In most case, string removal is relatively easy. This service is complimentary when tuning is also being done. Sometimes, the client can remove the string himself or herself.