Satava (2007) estimates that the costs of Columbus's 1492 voyage was 1,765,734 maravedís.
In today's money, what was the value of a 1492 Spanish maravedí?
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The author of this work has translated maravedis into dollars of 1929 by reference to statistics on purchasing power in wheat, corn and other staples. Hence his opinion that the maravedi was worth about two American cents of 1929.
According to the BLS inflation calculator, \$1 in 1929 is about \$15 today (2020).
So, if we accept Walsh's estimate, then one maravedi in 1492 converts to about \$3.00 today.
(And if we also accept Satava's estimate, then Columbus's voyage cost 1,765,734 $\times$ \$3.00 $\approx$ \$5,297,200 in today's USD.)
dtcm840's answer is about as good as you're going to do for a single actual number: it is clear, well-sourced, and almost certainly misleading. That's not their fault: all comparisons over time periods this far off are rendered meaningless by the very different markets & relative prices of commodities & labor between now and then. This question does not have an answer in the way you'd like.
I am suspicious of the Walsh source cited in dtcm840's answer: it used purchasing power based on staple foodstuffs for a basis for comparison; however food simply cost a far greater amount relative to other goods in the early modern period. An estimate based on the price of grain commodities between 1490 and 1920 will wind up dramatically undervaluing non-food goods.
This may account for why a recent reproduction of the Santa Maria alone, if we use Wilson's conversion, cost about 7 times as much as Satava's estimate for Columbus' entire voyage. Obviously, modern materials, and safety consideration, and they were building a museum, etc. But a factor of 7 times, for only the construction of just one of the ships? Clearly something isn't quite comparable here.
The potential for confusion with a simple conversion rate becomes particularly obvious if you look at things like wages, or the purchasing power available to actual people. A woman employed nursing foundling children in the period would've been paid about 100-200 maravedis per month, which on the Wilson estimate works out to an annual salary of \$750. Obviously this is an extremely low-status and correspondingly low-paid job, but it still highlights the difference between period and modern labor markets.
And that's why you can't really answer the question: it will only lead to confusion to say "A maravedi is worth about thirty cents" without also noting that a person's yearly labor was worth maybe a couple grand. Even saying that, conversion into modern currency doesn't help us understand that economy, because so much of Europe (and the world) was living under the current UN international poverty line, as an inevitable consequence of the different productivity levels of their economy and ours.
That's without getting into the implication that an iPad would have been about 2750 maravedis.
If you want to understand how much Columbus' voyage cost, you're better off comparing the price of the voyage to either the other activities undertaken by the Spanish state at the time, or to the total crown revenues, or something of this nature. Even then, recordkeeping is pretty spotty, and requires a lot of estimation--and estimates vary wildly. The answer here would probably be another question (and might be more appropriate for the history stack). If you are interested in the finances of the Spanish crown, this seems to be worth a read--although it'd have to be a closer reading than the one I've given it. One chart suggests that the Spanish military expenditures in 1565 were on the order of 1.8 million ducats, which by the 375 mrs to ducat conversion factor would be 675 million mrs, at which point Columbus' voyage is a rounding error. (Of course, at the 30-cent conversion rate, that would suggest Phillip II had a \$200-million annual military budget, roughly the amount Hitler sent Franco--a comment notes this is well into the inflationary period caused by importing New World specie; but even if prices tripled over those 70 years, it's still a rounding error in one year's military expenditures).
For another way to try to understand the magnitude of the outlay for this voyage, Vasco da Gama, the Portuguese admiral who opened the circum-African sea route to India, was awarded a royal pension of 300,000 reis upon his return in 1499. On this source, in the 1480s one Portuguese real was worth about 96/100ths of a maravedi, meaning da Gama's pension was roughly 285,000 mrs. (Contemporary conversions between currencies aren't perfect, but much less fraught than trying to create exchange rates 500 years apart.) So Portugal, one-fifth the size of Spain, was ready to pay the guy every 6 years what Satava estimates it cost to finance Columbus' entire voyage, just as a reward for prior services rendered.