Actually this is a very good idea. Gw figs and sets are geting more expensive each year by a little, but when you have to spend on the whole army, it becomes a fortune.
Luckily, the GW has provided the clue to cut the costs for those who can read between the lines.
In every set they give you more bits you can use, in fact they give you exactly twice as much. Ring a bell? You can make two units instead of one, if you know how.
Secondly, they have the sets in different price ranges, and this gives you the clue to the conversion priniciple:
First you buy the mother set in which you get one full unit, then you buy the cheapest set with which+ extra bits from the mother box, you can make the second half of the unit.
And also when building a unit you can plan in advance what will you do later on with the extra bits: For example, you can save the extra heads from the knights box for the greatswords, to have them look more stylish and also to save those extra heads from the greatswords for the later additions.
Let's try this with the example:
1. Combining the greatswords box with the statetroops box.
You go and buy greatswords box and build a complete unit, you can choose in this stage which arms-weapons will be saved for later on.
Then you buy state troops box and take all the bodies except those needed for the command group and build the second half of the greatsword unit with them ( maybe you still miss one or two models,) plus the extra arms-weapons from the greatswords box ( and the one from the state troops,) .
Optionally you can use those knights helmets for the original greatswords figs and the greatswords heads for the statetroopers, so they all look more stylish.
These state troops armed with great swords will then look like this unit champion
and so will probably have to be put in the front rank, so they look as they are charging the enemy. You have now a full unit of 20-18 greatswords instead of just 10 and you have wisely used all the bits from the greatswords box.
2.Combining the statetroops box and the freecompany box:
U still have the box of statetroops, so you only buy a free company box with 20 figs and together with the shields and weapons and possibly some helmets that were left from the previous conversion, you can build 10 swordmen and 10 halberds. One of them is full unit and one is missing the command group, but this can be added later on or if you build your army from the batallion box where you get 2 Greatswords command group sprues.
3. Combining the handgunners and free company boxes:
This is preety obvious: get the box of state gunners and the box of free company, combine them and build one unit armed with handguns and the other one with crosssbows. One unit can have full command and the other only the musician and the hero.
It's possible that I overlooked some extra bits in the boxes, since I'm building my army from the batallion box and there I got 2 command sprues among the greatswords, which gives me 2 banners, 2 musicians and 2 heroes options, which means I have an extra command group option for some other unit. The possibilities are really quite big it just takes some extra planning.
I leave to the rest of you, to calculate how much have we saved with all these, but any way it's not all about saving it's also having some good time with all these conversions, and giving some extra care and thought to your army.
I was typing this from my head, so It's possible, that I haven't got all the numbers right, so correct me, if you find any mistakes or better options.
.....and now I have to go and make myself some coffee.