No expert but I suspect even the more favourable exchange rate won't be enough to cover the rise in prices here since your last visit.
$100 a day will be enough depending on what you want to spend it on of course.For instance, the days of coming here to buy quality clothing at bargain prices stopped years ago. Now you have to pay over the top prices for even the shoddiest of goods.
Well if you have a place to stay and your ONLY major expense is food and entertainment, transportation is a minor expense, figure $5 for breakfast and $25 per meal average in a restaurant...! However if you eat a hotdog or empanadas you will have plenty money left.. Pizzas run up to $15..!! , Ah and night clubs figure $25 per person. Shows Theater... $30 tickets..!!
Now if you go to La Cabrera parrilla and enjoy Malbec, $100 will not be enough... for a day...
For one person? Good food and local transportation, though not nearly as cheap as 4 years ago, are still comparatively inexpensive. One can get along quite well on $100/day. That would allow good (not fancy) meals , plus the normal complement of Uber/taxis transportation, with an occasional non-touristic show, e.g. cafe Tortoni, Centro Cultural Borges, Torquato Tasso. Consult guiaoleo.com.ar - a decent Zagat like resource for restaurant info. You can lunch/dine well for a lot less than $25 per meal. Check out my fav local spot - Bar Norte on Talcahuano.
Moreover, if you're smart on breakfast and lunch (many places offer an executive lunch for about $10), you should be able to include a more touristic tango show ("drink" show, not dinner) at a decent club like Tango Porteno or Madero Tango.