〜ば〜のに details

Examples

if only — a present situation the unmet condition would still be fixing

もう少すこし家賃やちんが安やすければ、あの部屋へやに住すめるのになあ。

mou sukoshi yachin ga yasukereba, ano heya ni sumeru noni naa.

If only the rent were a bit lower, I could live in that flat. casual

あと一日いちにち休やすみが取とれれば、実家じっかにも寄よれるのに。

ato ichinichi yasumi ga torereba, jikka ni mo yoreru noni.

If I could take one more day off, I could stop by my parents' place too. neutral

you could have — the chance is already gone, and the main clause goes past

もう少すこし早はやく家いえを出でていたら、間まに合あったのに。

mou sukoshi hayaku ie o dete itara, ma ni atta noni.

If we'd left the house a bit earlier, we'd have made it. casual

天候てんこうが良よければ、予定よていどおり開催かいさいできたのに、本当ほんとうに残念ざんねんです。

tenkou ga yokereba, yotei doori kaisai dekita noni, hontou ni zannen desu.

If only the weather had held, we could have gone ahead as planned — it really is a shame. formal

ばよかった
ばよかった regrets the unmet condition itself and closes the sentence there; 〜ば〜のに spends its main clause naming the outcome that was lost, which ばよかった has no room for.
といいのに
〜ば〜のに spends its main clause naming the outcome the unmet condition would produce (もう少すこし安やすければ買かうのに); といいのに puts いい in that slot instead, so it names no outcome at all and only marks the condition itself as the thing wanted.

のに
The same のに, a different sentence — bare のに concedes a fact that DID hold (練習れんしゅうしたのに負まけた), while a conditional clause in front makes the whole sentence counterfactual: nothing in it happened.

たら
ば and たら both carry the counterfactual here; what fixes the reading is the tense of the MAIN clause — 買かうのに leaves a present situation unfixed, 買かったのに mourns a chance already gone. たら is the spoken default on a one-off past sequence.