There is a massive massive difference....
Both teams had hypothetical squads that cost 100m in either time period
Martinez wants a player that cost 30m.. he buys him and then has squad that cost 130m
Moyes wants a player worth 10m which we will say is back then's equivalent value.. He has to sell a 12m (35m) player to get him and has a squad that therefore cost 98m squad.. The other 2m subsequently disappearing.. lawn mowers.. Arteta money Bill? etc..
This happened many times, with only the odd occasion where we had positive spend actually worth mentioning.. we ended up with the equivalent of 1m a season spend.
But its not about how much more Martinez is spending than Moyes, as you say, its a different time now... its that he can just buy instead of breaking a bit of his squad to fix a different bit.
Martinez is short of striking options, he just spends 12m on a striker that doesn't get a game... meh whatever..
Moyes wants a striker, he has to work out whether to sell a quality defender or midfielder, or the youngster that is doing alright like Rodwell but is getting overhyped but may yet develop in order to get him..
Despite this, there is another huge difference between the two:
We improved under Moyes