I do not pay light in Brazil, if I want to build low cost renters with gpu, what would be a good configuration?
For Monero mining with GPUs, the best return on investment (ROI -- revenue divided by investment -- measured in months) is the Vega 56 or Vega 64 cards. It also produces the best revenue per watt for GPU mining for Monero. One card, even after today's big drop, can produce $170 / mo mining Monero. The cards cost anywhere from $500 - $750 depending on stock levels. Due to it's profitability in mining, cost has gone up and availability has gone down.
As for mining "in general", Nvidia GPUs are good for mining ZCash and BitcoinGold (ZEC and BTG). As of today, a $700-800 graphics card (Nvidia 1080ti) can earn $165 / mo with Zcash mining. This card has not been driven up in price much by mining, so if mining on it becomes less popular, it should hold more of its resale value than a Vega bought for $700+. On the other hand, the 1080ti is almost a year old. I assume nvidia will release a new card in the next 6 months, and this will cause the 1080ti to lose value as well.
The Nvidia 1060 card is probably the most flexible. It is profitable for mining zcash (and BTG which uses the same algorithm), Ethereum, and Monero. The 3gb card can cost as little as $200 each, and for zcash performs 90% as well as the $300 6gb model. I believe that Ethereum may need the 6gb model for the best performance moving forwards, due to an ever-growing blockchain.
For zcash, the 3gb model GTX 1060 offers the best profit margins. Other cards (1070, 1080, 1080ti) offer more revenue-per-watt when mining zcash, but the 1060 3gb offers the best revenue per dollar. For mining anything other than zcash (and BTG), only the 1060 and 1070 are appropriate. The 1080 and 1080ti perform poorly on other cryptos, being barely faster than the 1070, despite much higher prices.
Zcash performance on Nvidia cards (off the top of my head, alongside optimized power limits, and adjusting the clock speeds to stable higher values):
1060 3gb: 270 sol / s -- 85w TDP limit
1060 6gb: 300 sol / s -- 90w TDP limit
1070: 430 sol / s -- 120w TDP limit
1080: 500 sol / s - 130w TDP limit
1080ti: 700 sol / s - 200w TDP limit
This roughly follows the price of the cards:
1060 3gb: $200 and up -- $63 / mo revenue
1060 6gb: $280 and up -- $70 / mo revenue
1070: $400 and up -- $100 / mo revenue
1080: $500 and up -- $117 / mo revenue
1080 ti: $700 and up -- $164 / mo revenue
The only card in the above list that really beats $1 per sol/s is the 1060 3gb. Do keep in mind, with the lower revenue per GPU, you need to rely on motherboards with many pcie slots, and use riser cables, to generate meaningful revenue for each system you build.
Also keep in mind, these profit levels on these cards are unusually high. Between May 2017 and now (December 2017), only during May, June, and December has the revenue per card been this high. During July - October, it usually was in a range of $80 - $120 / mo for a 1080ti, and proportionally lower for the others. Don't assume profits will stay at this level.
I personally ignore any options that are not power efficient as I pay a lot for energy. Other people may be more familiar with low-cost high-revenue mining options that are less power efficient.
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Now, as a side tangent:
Even though you don't pay for power, the less power you use, the more you can mine on a given capacity power supply. Also, even with free power, your home only has so much power available to it. As your power is free, performance-per-watt clearly is less of a concern. But do still keep it in mind, especially if your options are similar in terms of ROI.
Some cards that are easier to get that may produce similar profits are the RX570 / RX580. They earn less, but they also cost less. However, they do use a similar amount of power to the Vega's, so you will need larger power supplies to earn the same amount of money, and they will generate a larger amount of heat per dollar revenue as well.
Also, I don't know about Brazil, but Venezuela was in a similar situation -- power is very cheap, which made mining very attractive. In Venezuela, the problem got so bad that the government started putting people in jail who were mining at home -- they considered it theft of electricity. Nothing in life is free. If you do mine with free or subsidized electricity, you still want to make sure you are using a low enough amount of it that it doesn't draw attention. If you want to stay under the radar, still better to use power-efficient hardware. More revenue for the same amount of power, and using less power draws less attention to you.
