Things like this have been suggested so many times especially buying inventory space with cash and they've been accepted multiple times too but never implemented, for example.
zarpgaming.com/index.php/forum/server-is...ver-on-ssrp?start=20
The plans we had when we accepted that suggestion were already implemented, which included the increase in the default inventory weight and addition of the inventory/shop/bank space character upgrades and bartering skill to increase shop weight! I'm definitely not against having more ways though, but it would have to be something more than a direct cash-to-weight method.