Here is the bad news, Costa Rica isn't really a budget destination. Budget £3,000 / $3500 per person (good independent 4* equivalent lodging & self drive) to £7,000 / $8000 per person (the best eco lodges and small aircraft transfers in peak season) for two weeks in Costa Rica including International Flights. Prices on the ground for meals and drinks are similar to what you would pay in California. Flights tend to be expensive from the UK and Europe and lodging and transport can also be expensive. Overall expect costs to be equivalent to travelling in Europe or the States.

Food & Drink

Prices at decent restaurants in Costa Rica are very sumilar too what you would pay in the US;

A simple meal at a local soda (cafe) is likely to cost in the region of $5 per person. 

A main course in a restaurant with a buzzing atmosphere in a tourist destination is likely to cost between $10 - $20 depending on how upscale it is.

A beer is likely to cost $2.50 for a local mass produced beer (e.g. Imperial) and $5 for a craft beer.

International Flights To Costa Rica

Flight costs vary depending on where you travel from. From the UK, expect to pay between £500 - £1,200 per person depending on tie of year and route. The British Airways direct flight from London is very popular and consequently very expensive, typically in the region of £1,000 per person. Flight prices during school holidays are always more expensive, however, you can often get a good deal if you fly via Miami (£650 per person if you are lucky). 

If flying from the US East Coast, United offer direct flights to San Jose, taking approximately 5 ½ hours, with prices from $515 return. Avianca fly with one stop and a 7 - 8 hour flight time, with prices starting at $370 return. From the West Coast, American Airlines, United, and Avianca offer a flight time of between 8 - 10 hours involving one stop. Prices range from $450 to $610 return. 

Accommodation

The best high end eco lodges are expensive and will add significant cost to your itinerary, typically in the region of $300 - $400 per person per night on a full board basis.  So, if you are on a tight budget, then unfortunately you will miss out on gems such as Pacuare and most of the lodges in the Osa Peninsula, such as Playa Cativa and Lapa Rios. High quality character 4* equivalent hotels (such as The Gilded Iguana, Hotel Capitan Suisso and Xandari) range from $100 - $200 per person per night depending on season and room category. 

There are plenty of mid range hotels in the region of $50-$100 per night per person, but these can be a very mixed bag particularly at the lower end of the spectrum. Typically in this price range, you can find some lovely spots with friendly staff, but furnishings tend to be a little old fashioned and many places struggle with the battle with damp in such a humid climate.

Self Drive vs Transfers vs Domestic Flights

The cheapest way get around Costa Rica is to self drive.  Driving in Costa Rica in most places is relatively easy if you are familiar with driving overseas. Some locations may involve dirt roads (e.g. Nosara and the last mile to many eco lodges), but on the whole it is good tarmac and easy going, especially if you are sticking to the mainstream destinations (Arenal, Tamarindo, Manuel Antonio, Papagayo Peninsula). Expect significant saving compared to transfers. Depending on where  you are heading, we will often recommend a high clearance AWD vehicle. Some of the eco lodges are reached only by boat (e.g. Pacuare, all lodges in Tortuguero and some lodges in the Golfo Dulce). We would usually get around this by organising to do these locations first with road (or air) and boat transfers and then arrange for your car to be dropped of to you at a rendez vous point so you can continue your onwards journey under your own steam.

If you are not confident about self driving, then having private transfers is the next step up. This can be either just point to point transfers (with the driver only with you for the transfers, fairly reasonable in price, but more expensive than self drive) or you can have an accompanying driver and private guide (a much more expensive option and usually unnecessary unless a group has very particular specialist interests or requirements).

Finally there is a good network of local scheduled small aircraft flights available. Costs are not outrageous on the scheduled routes, and if you are a family of 5 or more, then a private charter can also be cost effective. Obviously this is the most expensive option, but certainly the most convenient and the scenery is stunning. We would recommend though this is not suitable for those with a fear of heights, many of the routes can be fairly bumpy particularly where the Atlantic and Pacific weather systems meet.