1999 - one thousand, nine hundred and ninety nine yen.
2999 - two thousand, nine hundred and ninety nine yen.
Just under *3000* yen.
If you specifically want to say those exact numbers then the most direct way is to say them as stated above, however, it would be more natural to round the figure up to the nearest thousand, for example:
1999 - two thousand yen
2999 - three thousand yen.
If you want to specify that the amount is under the rounded up figure, you can say 'Just under 3000 yen' in place of 2999 yen.
数字をきっちりと言いたいなら、上記のように言えます。ただ、千単位にした方がより自然だと思います。例えば：
1999 - two thousand yen
2999 - three thousand yen
千単位にした場合、それよりも少ないことを：
Just under 3000 yen（3000円ちょっと切れる）
↑のように表せます。
2999 -- both "two thousand nine hundred ninety nine" AND "twenty nine ninety nine"
I think some native speakers might use "nineteen ninety nine" and "twenty nine ninety nine" because, in our home countries, we see prices like this all the time. For example, if you hear an ad on the TV in the US, and the price is $29.99, you will likely heard someone say "twenty nine ninety nine", because it's shorter than saying "twenty nine dollars and ninety nine cents".
「nineteen ninety nine」「twenty nine ninety nine」と言うネイティブもいるかもしれません。それは、いつも自分たちの国で、こういう値段を見ているからです。例えば、アメリカのテレビの宣伝で、値段が「$29.99」だったら：
"twenty nine ninety nine"
↑こう言うことが多いと思います。この方が「twenty nine dollars and ninety nine cents」より短いですから。
When writing down a number from a digits form into a worded form in English we always begin with the larger digits first and finish with the smallest digit.
For example, If i wanted to word 152, I would say 'one hundred and fifty two'. Starting with the hundred which is the greater number than the fifty two.
or an even bigger number like 11368 would be 'eleven thousand, three hundred and sixty eight'. So here I am dealing with the greater number of eleven thousand first and finishing with the smallest number of sixty eight.
数字を文字にする場合、桁の大きな方から始めます。
例えば、「152」を文字にするなら：
'one hundred and fifty two'
と言えます。「fifty two」よりも大きな「one hundred」から始めます。
もっと大きな数字、例えば「11368」なら：
'eleven thousand, three hundred and sixty eight'
より大きな「eleven thousand」で始めて、一番小さな「sixty eight」で終わっています。
When the sum is almost a round figure, you can 'round up' the price for convenience. So, 1999 yen is 'Two thousand yen'. The 1 yen is insignificant in most conversations.
1999 - one thousand nine hundred and ninety nine yen
2999 - two thousand nine hundred and ninety nine yen
When talking about numbers we always say the largest number first so one thousand/two thousand
when talking about money sometimes we round it up to the nearly hundred/thousand if there only a few pence in it so you could say - nearly 2 thousand yen for 1999 or nearly three thousand yen for 2999
1999: "nineteen ninety nine yen" OR ""one thousand nine hundred and ninety nine yen".
2999: "two triple nine yen" OR "two thousand nine hundred and ninety nine yen".
When you want to express that something is 1999 or 2999 yen, then you may say it in the following ways:
-1999: "nineteen ninety nine yen" OR "one thousand nine hundred and ninety nine yen".
-2999: "two triple nine yen" OR "two thousand nine hundred and ninety nine yen".
1999円あるいは2999円は、次のように表せます。
-1999: "nineteen ninety nine yen" もしくは "one thousand nine hundred and ninety nine yen"
-2999: "two triple nine yen" もしくは "two thousand nine hundred and ninety nine yen"
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