Are Centrica plc, Just Eat plc and Sirius Minerals plc simply too expensive?

The announcement of half-year results from Centrica(LSE: CNA) this morning saw the shares hit a new high for the year of 248p in early trading.

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The headline numbers were less than impressive. Revenue was down 13%, while underlying operating profit and earnings per share (EPS) were 12% and 17% lower respectively. This was largely as a result of low commodity prices continuing to take their toll on the group's upstream business plus extreme warm weather in North America leading to lower demand at the consumption end of the business.

However, the turnaround of the company under Ian Conn -- chief executive since January last year -- appears to be progressing satisfactorily. Indeed, Centrica today upped its cost savings target for 2016 to £300m from the previous £200m.

Nevertheless, the owner of British Gas is at a relatively early stage of reshaping the group into "a robust platform for customer-focused growth." As such, a forward price-to-earnings (P/E) ratio of 16.3 looks a little too pricey to me at this stage, although a 5% dividend yield may be attractive to income investors.

Tasty growth share

Just Eat(LSE: JE) has delivered tremendous top- and bottom-line growth as it has rapidly become the world's leading online and mobile marketplace for takeaway food. There was more of the same in the company's half-year results today, with revenues up 59%, underlying earnings before interest, tax, depreciation and amortisation( EBITDA) up 107% and underlying EPS up 81%.

As a result of the first-half performance, and the company's "very strong position both operationally and financially," management has increased its forecasts above the market consensus for the full year. Revenue is now expected to be £368m (up from £358m), with underlying EBITDA between £106m and £108m (up from £102 to £104m).

The shares are riding at an all-time high of over 500p, putting the company on what I estimate is a forward P/E of around 45. That may seem expensive, but I reckon a cheap price-to-earnings growth (PEG) ratio of 0.7 makes Just Eat an attractive buy.

Speculative investment

Sirius Minerals(LSE: SXX) currently generates no revenue, let alone profit, and but for the fact that it owns the world's largest and highest-grade polyhalite deposit with a potential life of over 100 years, I wouldn't give it a second look.

As it is, I've written about Sirius several times as a worthwhile speculative buy. In April, the shares were trading at 15.5p, valuing the company at £355m. However, they've had a strong run up to a current 26.5p and the value is now £611m. So, are the shares still worth buying?

Last month, Sirius reported significantly reduced capital funding requirements for the project and "positive progress on the company's financing plans with a number of parties undertaking detailed due diligence."

I reckon the equity component of the financing will amount to around £400m, giving a theoretical market cap of just over £1bn at the current share price. As we're looking at £3bn annual revenues and industry-leading margins (albeit not until the mine is built), and as some industry analysts reckon the shares should be up around 40p today, based on discounted cash flow models and suchlike, I believe Sirius Minerals is still a worthwhile speculative buy at the current price of 26.5p.

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G A Chester has no position in any shares mentioned. The Motley Fool UK has recommended Centrica. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.