I do indeed mean all 8 at the same time. Plus Gathering (MIN/BOT/FSH). Would be the best and cheapest way to go. Crafting classes have good cross skills abilities (Hasty Touch for example is an excellent early cross class ability that once learned through CUL can be used in every crafting class from level 15). The first 15 levels in all crafting classes are easy as the resources required can be bought from the crafting guilds. The items made and sold back will cover the cost of those materials and you should end up with a slight profit. If you know someone who can craft you could get them to make you some "levekits". This will pretty much boost you along the crafting route up to 50 very easily.

Raw materials can be a good quick way to make gil, try silver ore if you can mine it as it is used in low level crafts and end game crafts.

Also make sure you're leveling up your retainers and sending them on ventures to bring back extra goodies.

If you know anyone who plays tank / healer you could dungeon grind a lot faster as they can pop the queue for you. Hopefully you're in a good FC that can help you out and get you geared. If there is a crafter within the FC see if they will craft you items if you bring them the materials. The rare components are usually bought with Tomes and so that could reduce the cost.