I think promoting a post is stupid on Steemit.
I understand that you are trying to grow your account by trying to get new followers and the quickest way is to promote it into trending or hot.
I think that if your post is good enough you shouldn't have to do this. We all know it is hard enough to earn rewards on your posts so why would you waste the money required to get it to trending.
If we look at the trending posts they are between $350 and $500. If we calculate what is paid for by the author and what is paid for by followers it doesn't make sense. Of the $500 on a post you will quite often see $480 has been spent by vote bots and the author has actually lost money.
I know that if you use smart steem and pay for votes you get $11.2 per $10 spent. If you take curation into account which is 25% of the total then for every $10 you spend you are losing $1.60 as 75% of $11.2 is $8.40.
Therefore on a $500 post an author could be losing as much as $80 per post. If someone offered me $80 I would much rather power up and gain followers organically than going into trending.
There is always a risk of receiving flags for promoting a post that doesn't seem good enough. It happens regularly that a post will be flagged for abusing the system. This is good as only quality pieces should be in trending and this is hardly the case today.
Depends on what you want to achieve.
HOT or TRENDING?
Your question is a hard one to answer because these numbers vary, a lot. Depending on the price of STEEM, activity on the blockchain, competition in the tag, etc.
But, let's look into how much it currently takes to get where you want to be.
It is much easier and cheaper to get into HOT than it is to get into TRENDING. For HOT an article needs to get 20-50$ in upvotes in a matter of the first 1 - 3 hours since its posted. To get there without any organic upvotes, just through paying bidbots for promotion one would need to spend around 10-25 STEEM/SBD.
As for TRENDING depending on the position you want to take, and the competition in the tag/overall. To get into trending an article needs to get around 200-700$ in upvotes. If you want to achieve this without any organic upvotes, you would need to spend at least 100 up to 350 STEEM/SBD to get there. The more you spend though, the higher you rank at the trending page.
Currently, the top TRENDING post has more than 700$ in upvotes. One would need to spend about 400 STEEM/SBD in order to get in the first place with more than 700$ in bidbot upvotes.