Don't buy turnips for more than 150, that is too much.If you have a dump of money, I'd say go for it. If you know a time traveler who can help you get to amazing prices, go for it.
Don't buy turnips for less than 75, that is too less.If you have A LOT (I mean a lot.) of money, go ahead!
Check Re-Tail prices.There are two prices: One when it opens, and one afternoon and onward. If the afternoon price is higher than the previous price for that day, you may have multiple spikes in the middle on the week. If you have a really high price, go for it! It's now or never.
Check your friends' prices!Your friend's price may be higher! Also, look here on TBT, there will be amazingly high prices sometimes.