I would have to lean towards the droid army. The development of all of the systems, vehicles, let alone production, must have cost incredible amounts. Also, the sheer amount of them was large, and they were in service long before the clone wars for the various Unions that had them. The wiki states that a Lucrehulk class battleship can hold over 329,000 B1 Battle Droids. Research on the production of Lucrehulk class battleships shows that at the low end 1,000 were produced and at the high end 24,000 were produced. So, assuming that every Lucrehulk was fully loaded(a fair assumption given the might of the organizations that were supplying droids), and looking at a mid range figure for the production of Lucrehulks (10,000), you would have a B1 strength of 3,296,000,000. You would also have 15,000,000 vulture droid starfighters, 500,000 C-9979 landing crafts, 62,500,000 AATs, 5,500,000 MTTs, along with the other droid types (B2 Super Battle Droids, Droidekas, MagnaGuards, and so on and so forth.) You can see that there was an unimaginable amount of droids that the Separatists had to play with. Take note that this was just with 10,000 Lucrehulk Battleships, and the Separatists have many other sources of droids and cruisers and frigates that all can carry droids and that did carry droids. But, to show you the hopelessness of the Republic, we will use the B1 strength in only the Lucrehulks. The actual figure was probably in the trillions.
The original order for clones was 3,000,000 and another order was made later in the Clone Wars. However, they were not all produced during CW, and <PERSON> tells <PERSON> that they have created 200,000 units with millions more on the way. So, even if we are generous and include all 3,000,000 clones in the cost, then a clone would be about 110 times more expensive than a droid for the clone army to be more expensive than the droid army.
Sorry, did I say 110? I meant 110,000. 126
Yeah. A droid's blaster alone costs 900 credits (Thanks, Wookiepedia). So, a generous estimate for the cost of a B1 would be 2000 credits. So, for the clones to be more expensive than the droids, each droid would have to cost 219,733,333 credits. If you are not from the Star Wars Universe, that is a lot. Like a lot. Like buy a planet a lot.
Basically, per unit, for the armies to cost the same, a droid would have to cost 219,733,333 credits. Now, REMEMBER, that is just considering the B1 droids that were on 10,000 Lucrehulk battleships.
For the next part of the question, whether cost was a deciding factor, the answer is no. Since no one knew that Palpatine had purchased the clone army (see the bit below), cost was not a deciding factor. And, for the Seps, since the organizations that produced battle droids were part of the Seps, cost was not a factor.
I’m pretty sure that’s it, feel free to suggest any edits. Hope it helped!
Note: In my cost calculations, I did not begin to account for cruisers/starfighters/frigates. The amount of math would be incredible, seeing all the types of spaceships, but it is fair to say that the Seps would still leave the Republic in the dust in quantity.
To the comment above, (I don't have 50 reputation), when <PERSON> became Supreme Chancellor, he took funds from the public and sent them to Kamino secretly, so the cloning facilities got paid by The Republic, and since Sifo-Dyas had ordered the clones, it looked good for Kamino. 963