#274 Remove duplicated code for loading post price

@mturalenka Review the code, please.
in case of `Persistent_Caching` this can cause an issue when post type was changed but cached
@drdla should we take care about this?
if yes, solution using singleton pattern will be enough:
```
if (empty(self::$key)){
    self::$key = 'new value';
}
return self::$key;
```