The simple past went up is not wrong, but it relates to what happened at some time in the past, and it is normal (but not essential) to specify the time in the past.
The cost of gas went up in December, but it's going down again.
The cost of gas went up recently, but it's going down again.
You can use present perfect to talk about something that happened in the past if some effect persists at the time of writing. With present perfect, you never specify when the thing happened. For example, if the price of gas went up in December and it's stiil high (above the pre-December level), you can say
The cost of gas has gone up, but it's going down again.
Note that it's going down again is (present continuous) can mean one of two things:
- the price of gas started going down recently, is still going down at the time of writing, and is expected to continue doing so
- the price of gas is expected to go down at soem time in the future
For the first meaning, you must not specify when: for the second meaning, specifying when is optional
The cost of gas has gone up, but it's going down again in February.
The cost of gas has gone up, but it's going down again soon.
If the price has already gone down again and is expected to stay at the current level of the foreseeable future, you can say:
The cost of gas went up, but it went down again.
It would be normal to specify when the price went up and when it went down, but it is not essential.